How to Respond to a Patent Cease & Desist Letter

If you've just received a patent cease and desist letter, take a breath. Your first instinct might be to panic, but you have time to respond properly. Understanding how to respond to a patent cease and desist letter strategically—rather than reactively—can make the difference between a costly legal battle and a favourable resolution.

In this guide, we'll walk through what happens when you receive a patent infringement letter, your timeline for response, and where technical analysis fits in. We've supported law firms and patent holders through dozens of these situations and know what works.

What Is a Patent Cease & Desist Letter?

A cease and desist letter (also called a patent threat letter or demand letter) is a legal notification claiming your product or service infringes someone's patent rights. Critically, it's not a lawsuit—it's a pre-litigation demand.

Typical contents include:

• Patent number(s) and claims allegedly infringed
• Description of your accused product or service
• Demands: stop selling, licensing agreement, damages, or all three
• Response deadline (often 14-30 days)
• Threat of litigation if demands aren't met

The letter is designed to create urgency and pressure. That's intentional. But receiving a cease and desist letter doesn't automatically mean the claim has merit. In our experience, many allegations collapse under technical scrutiny.

According to research, approximately 85-90% of patent disputes that reach litigation settle before trial, often because the technical evidence reveals weaknesses in either the infringement claim or the patent's validity.1

Your Timeline: How Long You Have to Respond

UK Jurisdiction: There's no statutory response deadline for a cease and desist letter itself in the UK. However, under the Civil Procedure Rules (CPR), formal pre-action protocols require a "letter before action" stage to allow negotiation.2 Response deadlines stated in the letter (commonly 14 days) are often the sender's preference, not a legal requirement. That said, ignoring it entirely creates problems.

If proceedings are commenced in the Intellectual Property Enterprise Court (IPEC), cases typically aim for trial within 12 months, though delays occur.3 The Patents Court operates on similar timelines for higher-value disputes.

US Jurisdiction: In the US, response deadlines vary widely—anywhere from 7 to 30 days is common. Unlike the UK, there's no formal pre-action requirement in US federal patent litigation. The sender can file a complaint whenever they choose.4 However, responding by the stated deadline demonstrates good faith and can influence a court's view if litigation follows.

The Reality: The urgency expressed in most cease and desist letters is exaggerated. You typically have enough time to engage proper legal and technical counsel, preserve evidence, and assess the claim methodically. Rushing leads to poor decisions.

What NOT to Do (Critical First Steps)

Before we discuss what to do, here's what you must avoid:

Don't Ignore It

Ignoring a patent infringement letter can be used against you in court as evidence of bad faith or willful continuation of alleged infringement.5 Even if you believe the claim is baseless, acknowledge receipt and indicate you're evaluating it with counsel.

Don't Immediately Comply or Admit Anything

Just because someone sent you a letter doesn't mean their claim is valid. Don't stop selling your product, agree to licensing terms, or admit infringement without proper analysis. Legal language and aggressive tone don't guarantee a strong case.6

Don't Respond Emotionally or Too Quickly

A rushed email or angry reply can shift leverage to the other side before litigation even begins.7 Every word in your response becomes part of the legal record. Take time to craft a proper response with your legal team.

Don't Destroy or Modify Evidence

Preserve everything: accused product versions, source code, documentation, development history, and internal communications about the product. If you modify your product after receiving the letter, that can complicate your legal position. Technical experts will need access to exactly what existed when the letter arrived.

Don't Make Public Statements

Avoid discussing the dispute publicly, on social media, or with customers until you've consulted counsel. What seems like defending your reputation can create legal complications.

What to Do Immediately

Here's your action plan for the first few days:

1. Engage Patent Litigation Counsel

Contact a solicitor or attorney specializing in patent litigation immediately. This isn't general commercial law—patent disputes involve complex technical and procedural questions. In the UK, look for firms with experience in IPEC and Patents Court proceedings. In the US, seek counsel experienced in federal patent litigation.

2. Preserve All Evidence

Create a complete archive of:

• The accused product or service (exact version in operation when letter received)
• Source code, firmware, technical specifications
• Development documentation and version history
• Marketing materials and user guides
• Internal communications about product development
• Any prior contact with the patent holder

Mark everything as potentially subject to litigation hold. Technical experts will need this material for claim chart analysis.

3. Review the Patent and Claims

Obtain the full patent specification from the UK Intellectual Property Office, European Patent Office, or USPTO (depending on jurisdiction). Read it carefully. Patent claims are structured hierarchically:

• Independent claims define the broadest scope of the patent
• Dependent claims narrow that scope with additional limitations

Focus on the specific claims cited in the letter. These are what you'll need to assess for infringement.

4. Initial Assessment

Ask yourself:

• Does this patent holder have a history of enforcement? (Check publicly available litigation databases)
• Is this an operating company using the patented technology, or a non-practicing entity (NPE)?
• How specific is the letter about the alleged infringement?
• What's the commercial context—are we competitors, or is this purely licensing-driven?

These questions help assess whether this is a credible threat or a speculative fishing expedition.

Assessing the Claim: Do They Actually Have a Case?

Not every patent infringement allegation has merit. Two fundamental questions determine the strength of the claim:

1. Does the patent actually cover your product? (Infringement analysis)

2. Is the patent even valid? (Validity analysis)

This is where technical analysis becomes critical.

The Role of Technical Experts

We've analysed dozens of patent disputes, and in our experience, many allegations fail when subjected to rigorous technical scrutiny. Here's how technical experts contribute:

Claim Chart Analysis

A claim chart maps each element of a patent claim to specific evidence in the accused product. Think of it as a detailed checklist where every box must be ticked with proof.8

For example, if Claim 1 reads "a method for filtering content using a trained neural network comprising: (a) receiving input data; (b) processing data through a neural network trained on a dataset; (c) outputting filtered results," the claim chart must show evidence that your product performs each step.

Critically, patent infringement requires every element of the claim to be present.9 If even one element is missing or implemented differently, there's no literal infringement. Technical experts review your actual implementation—source code, system architecture, product behaviour—to determine what's genuinely there versus what the patent holder assumes.

Prior Art Searching

Just because a patent was granted doesn't mean it's valid. Patents can be challenged through prior art—evidence that the invention was already known before the patent's priority date.

Technical experts conduct prior art searches in academic databases, technical publications, open-source repositories, and product documentation. Finding prior art that describes the same method or system can invalidate the patent entirely.10

According to USPTO data, prior art is frequently discovered during litigation that wasn't available during initial patent examination, making invalidity challenges a viable defence strategy.11

Source Code Review

For software patents, technical experts review the actual source code to understand implementation details. Patent claims use functional language ("a method for..."), but code reveals exactly what happens. Often, products achieve similar outcomes through fundamentally different technical approaches.

Non-Infringement and Invalidity Opinions

Based on claim chart analysis and prior art research, technical experts prepare formal opinions that serve critical legal and strategic functions.24

Non-infringement opinions assess whether your product or activity infringes the patent through detailed technical evaluation and claim-by-claim analysis.25 These opinions can defend against willful infringement allegations by demonstrating you acted with due diligence and good faith.26

Invalidity opinions document prior art analysis and technical reasons why the patent may not satisfy patentability requirements. While invalidity opinions cannot formally challenge a patent outside of proper legal proceedings, they provide crucial documentation of diligent investigation and can support settlement negotiations.27

These opinions form the technical foundation of your legal response. When supported by rigorous technical analysis, they often lead patent holders to reassess their position or engage in meaningful settlement discussions.

Your Response Options

Once you've assessed the claim with legal and technical counsel, you have several strategic options:

1. Challenge Infringement

If claim chart analysis reveals your product doesn't contain all claim elements, you respond with a non-infringement position supported by technical evidence. Detailed technical analysis often clarifies the distinctions between the patent claims and your actual implementation, allowing both parties to reassess the situation based on facts rather than assumptions.

2. Challenge Validity

If prior art research uncovers invalidating references, you can challenge the patent's validity. Under the UK Patents Act 1977 Section 72, patents may be revoked on grounds including lack of novelty or inventive step (often based on prior art).17 Validity challenges can be raised as counterclaims in infringement proceedings before the Patents Court or IPEC, following the procedures set out in Practice Direction 63A.18

In the US, the Patent Trial and Appeal Board (PTAB) offers two post-grant challenge procedures: Inter Partes Review (IPR) for prior art challenges, and Post-Grant Review (PGR) for broader validity challenges—both providing alternatives to district court litigation.1920

3. Negotiate Licensing

If technical analysis confirms the claim has merit, licensing may be the most practical path. This doesn't mean accepting the sender's initial demands—licensing negotiations typically involve multiple phases of preparation, discussion, proposing, and bargaining over scope, royalty rates, and terms.21

Accurate patent valuation is essential for negotiating favourable terms.22 Technical experts contribute by analysing the market landscape, competitive positioning, and how central the patented technology is to your product's functionality and revenue.23 This evidence-based valuation strengthens your negotiating position and helps counter attempts to overvalue the patent or demand unreasonable royalty rates.

4. Design Around

If infringement is confirmed but you want to continue selling your product, a design-around strategy modifies your implementation to avoid infringing claim elements. This requires careful technical analysis—under patent law, removing or substantially changing even one claim element can avoid infringement.12

Design-around is particularly effective when the patent claims are narrow or when alternative technical approaches exist that achieve the same commercial outcome.

5. Prepare for Litigation

If the sender is serious and settlement isn't possible, prepare for formal proceedings. In the UK, this means IPEC (for claims up to £500,000 in damages with costs capped at £60,000) or the Patents Court for higher-value disputes.13 In the US, federal district court litigation is significantly more expensive, often costing hundreds of thousands to millions of dollars.

Each option requires different technical evidence. Early engagement of technical experts allows you to pursue the strategy with the strongest evidentiary foundation.

What Happens After You Respond

The sender's next move depends on the strength of your response and the underlying merits of their claim.

If You Respond with Non-Infringement or Invalidity Position

When presented with well-reasoned technical analysis, patent disputes frequently resolve at the pre-litigation stage. Empirical research shows that the majority of patent disputes settle rather than proceed to trial, with settlement rates varying by forum and case complexity.2829

If your response includes credible technical analysis, patent holders may:

• Withdraw the claim entirely
• Narrow their allegations to specific claims or product features
• Propose licensing on revised terms
• Proceed to litigation (if they believe they'll prevail despite your evidence)

Settlement likelihood increases when uncertainty about litigation outcomes is reduced through clear technical evidence.30 A well-documented non-infringement or invalidity position can significantly shift the negotiation dynamics.

If You Don't Respond

Ignoring a patent infringement letter is not advisable and can prove costly.31 Upon receiving actual notice of a patent claim, you have an affirmative duty to exercise due care to investigate whether infringement is occurring.32

Failing to respond can lead to:

• Willful infringement findings: Courts may determine you acted in bad faith by ignoring the claim
• Enhanced damages: Willful infringement can result in treble (triple) damages and attorney fees33
• Loss of defenses: You lose the opportunity to establish good-faith reliance on competent analysis

Some senders may never follow through with litigation, but there's no reliable way to predict this. Even if you believe the claim lacks merit, acknowledging receipt and indicating you're evaluating it with counsel protects your position.

Timeline to Litigation

UK: If the sender intends to litigate, they'll typically issue proceedings in either IPEC or the Patents Court. Cases aim for trial within 12 months, though this often extends in practice.3

US: Patent holders can file a complaint in federal district court at any time. From complaint to trial, cases typically take 18-24 months, though most settle earlier.1

Unjustified Threats (UK-Specific Advantage)

Here's something many recipients don't know: under the Intellectual Property (Unjustified Threats) Act 2017, patent holders face risks when sending cease and desist letters in the UK.14

If a threat of patent infringement proceedings is unjustified (the patent claim fails or is groundless), the recipient can bring a counterclaim for unjustified threats. Remedies include injunctions, damages, and declarations that the threats were unjustified.

Certain communications are "permitted" under the Act—such as simply notifying someone that a patent exists.15 But aggressive demand letters threatening litigation can cross into actionable territory if the underlying claim is weak. This creates a deterrent against speculative or abusive cease and desist letters in the UK.

Costs & Practical Realities

Patent disputes are expensive. Understanding the costs helps you make informed decisions.

Cost of Responding (Legal + Technical)

UK: Initial response preparation (legal + technical analysis) typically ranges from £10,000 to £30,000 depending on complexity. Claim chart analysis, prior art searches, and expert opinions all require specialist time.

US: Similar initial work costs $15,000 to $50,000 USD.

Cost of Litigation

UK IPEC: Costs are capped at £60,000 for liability determination and £30,000 for damages proceedings.13 This makes IPEC relatively accessible for small and medium-sized enterprises (SMEs).

UK Patents Court: No cost cap. Full litigation can cost £200,000 to £500,000+ depending on case complexity.

US: Federal patent litigation averages $1 million to $5 million through trial, depending on case complexity and amount at stake.16

Security for Costs (UK-Specific Consideration)

If you're defending a patent claim in the UK, particularly if you're based outside the jurisdiction, you may face a security for costs order. Under Civil Procedure Rules Part 25, the court can require you to deposit money or provide security (such as a bank guarantee) to cover the patent holder's potential legal costs if you lose.3435

Security for costs is particularly common when:

• The defendant is resident outside the UK
• There are concerns about the defendant's ability to pay costs if unsuccessful
• The case involves significant legal expenses

The amount ordered depends on the estimated costs of the litigation. In Patents Court cases without cost caps, security orders can run to hundreds of thousands of pounds. Even in IPEC, where costs are capped at £60,000, security orders can still represent a substantial additional financial burden beyond your own legal costs.36

This is an important consideration when deciding whether to defend a claim or negotiate settlement—you may need to fund both your own defence AND provide security for the other side's costs before the case proceeds.

Cost of Ignoring vs. Responding

Ignoring may seem cheaper initially, but if litigation follows, you've lost critical opportunities.37

Pre-litigation resolution: Early response enables settlement or licensing discussions before formal litigation commences, potentially saving significant costs and time.38 Patent litigation is expensive and time-consuming and should not be undertaken lightly when pre-suit resolution remains possible.39

Good faith demonstration: Courts consider whether you responded appropriately to patent allegations when assessing willfulness.40 Having a substantial or legitimate defence to infringement—documented through proper analysis—is the strongest predictor of avoiding willfulness findings.41 Responding with well-reasoned technical analysis demonstrates you acted responsibly rather than deliberately ignoring known risks.

Strategic positioning: Taking time to understand the patents, assess your position, and gather evidence allows for sound business decisions rather than emotional reactions forced by litigation deadlines.42

Business Continuity

Can you continue selling your product while this is unresolved? Generally yes, unless a court grants an interim injunction.43

UK: Courts apply the American Cyanamid test for interim injunctions, which requires demonstrating a serious issue to be tried, inadequacy of damages, and balance of convenience favouring an injunction.44 In patent cases, interim injunctions are often not granted because patent holders can typically be compensated through damages for infringement occurring before trial.45

US: Following the Supreme Court's eBay decision, patent holders must demonstrate irreparable harm, inadequacy of monetary damages, balance of hardships in their favour, and that public interest supports an injunction—there is no automatic presumption of irreparable harm.4647 Companies routinely continue selling accused products throughout patent litigation unless an injunction is granted.48

However, patent disputes create commercial uncertainty. Customers may hesitate to purchase, investors may have concerns, distribution partners may seek assurances, and your team will spend time on legal matters rather than product development. Managing this business impact is as important as the legal strategy.

Insurance

Some businesses have intellectual property insurance that covers patent infringement defence costs.49 IP insurance policies typically offer several types of coverage, including defence coverage for legal costs when defending against patent infringement allegations.5051

Before-the-event (BTE) insurance can be obtained before any infringement claim arises and is generally less expensive. After-the-event (ATE) insurance can sometimes be obtained even after proceedings have commenced, though with higher premiums.52

Companies with infringement defence insurance have reduced patent defence costs by 50% to 75% in reported cases.53 Given that the average defence of a moderate-sized patent case costs approximately $2.9 million before settlement or damages, insurance coverage can significantly reduce financial exposure.54

Check whether your existing business insurance includes IP defence coverage, or consider obtaining specific IP insurance if you operate in patent-intensive industries.

Conclusion: Strategy Over Panic

Receiving a patent cease and desist letter is serious, but it's manageable with the right approach. You have time, options, and resources available to respond strategically rather than reactively.

The key takeaways:

Immediate priorities:

1. Engage specialised patent litigation counsel immediately—this isn't general commercial law
2. Preserve all evidence: products, code, documentation, development history
3. Don't ignore the letter (risks willful infringement findings), but don't admit anything either
4. Bring in technical experts early for claim chart analysis and prior art research

Strategic considerations:

• Technical analysis is critical: Whether the patent actually covers your product and whether it's even valid are questions that require rigorous technical evaluation, not assumptions
• Good faith matters: Courts consider whether you responded appropriately when assessing willfulness—having a substantial, well-documented defence is the strongest predictor of avoiding enhanced damages
• Pre-litigation resolution is possible: Most patent disputes settle, and early technical evidence often shifts negotiation dynamics significantly
• Cost planning is essential: Beyond legal fees, consider security for costs (particularly if you're outside the UK), potential business disruption, and whether you have IP insurance coverage

UK-specific advantages:

• IPEC offers cost-capped litigation (£60,000) for smaller disputes
• Unjustified threats provisions protect recipients of weak or groundless patent claims
• Interim injunctions in patent cases are uncommon—you can usually continue selling your product

We've supported law firms and patent holders through dozens of these situations. In our experience, panic leads to poor decisions—accepting unreasonable demands or ignoring legitimate claims. Methodical technical analysis, combined with strategic legal counsel, leads to the best outcomes: either demonstrating non-infringement clearly, finding invalidating prior art, negotiating fair licensing terms, or preparing a strong litigation defence.

If you're facing a patent dispute and need technical analysis—claim charts, prior art research, or source code review—we can help. Our First consultation is free. Get in touch to discuss your situation.

---

Sources

1. Receiving a Cease & Desist letter – Part 1: Where to start — Patent litigation settlement statistics
2. Patent infringement action—flowchart — UK civil procedure for patent disputes
3. IPEC Guide — Intellectual Property Enterprise Court procedures and timelines
4. Responding to a Patent Notice Letter: Key Considerations — US patent cease and desist response guidance
5. How to Respond to a Cease and Desist Letter Without Losing Leverage — Common mistakes in responding to demand letters
6. What to Do If You Receive a Cease-and-Desist for IP Infringement — Evaluating cease and desist letter credibility
7. What to do if a patent owner sends you a cease-and-desist letter — US patent demand letter response best practices
8. Claim Chart — Claim chart methodology and analysis
9. Claim chart - Wikipedia — Overview of claim chart types and infringement requirements
10. MPEP Section 2121 - Prior Art; General Level of Operability Required — USPTO guidance on prior art standards
11. Search for patents - USPTO — Patent search resources and tools
12. How to Design Around and Avoid Patent Infringement — Design-around strategy and methodology
13. Intellectual Property Enterprise Court—costs capping in practice — IPEC cost caps and damages limits
14. Intellectual Property (Unjustified Threats) Act 2017 — UK unjustified threats legislation
15. Manual of Patent Practice - Section 70B - Permitted Communications — UK IPO guidance on permitted communications
16. United Kingdom : Patent Litigation — UK patent litigation costs and procedures
17. Patents Act 1977 - Section 72: Power to revoke patents on application — UK statutory grounds for patent revocation
18. Practice Direction 63A – Intellectual Property Claims — UK civil procedure rules for patent validity challenges
19. Inter Partes Review - USPTO PTAB — US post-grant review procedure for prior art challenges
20. Post Grant Review - USPTO PTAB — US post-grant review procedure for broader validity challenges
21. Exchanging Value – Negotiating Technology Licensing Agreements: A Training Manual - WIPO — Authoritative guide on patent licensing negotiation phases and best practices
22. Patent Valuation for Licensing: Strategies and Tips — Patent valuation methodologies for licensing negotiations
23. Understanding Royalty Rates in Patent Licensing — Royalty rate determination and licensing valuation approaches
24. Patent Opinions and Expert Insights - O&R Patent Law — Overview of patent opinion services and their legal applications
25. Patent Non-Infringement Opinion - Practical Law — Authoritative guide on non-infringement opinions and freedom to operate analysis
26. Using Patent Opinions to Avoid Litigation and Spur Strategic Growth - Taft Law — Role of patent opinions in willfulness defence and enhanced damages avoidance
27. Invalidity Opinions of Counsel After Commil - Fredrikson & Byron — Legal role and strategic value of invalidity opinions in patent disputes
28. How are Patent Cases Resolved? An Empirical Examination - SSRN — Empirical study of patent litigation outcomes and settlement rates
29. Section 337 Statistics: Settlement Rate Data - USITC — Patent dispute settlement and withdrawal statistics
30. Do Standard-Essential Patent Owners Behave Opportunistically? - SSRN — Research on settlement dynamics and factors influencing patent dispute outcomes
31. Cease-and-Desist Letters: What to Do When Your Business is Accused Of Patent Infringement - McLane Middleton — Legal consequences of ignoring patent infringement letters
32. Know the Law: Can My Business Ignore a 'Cease and Desist' Order for Patent Infringement? - McLane Middleton — Due care requirements after receiving patent infringement notice
33. What to Do When Your Business is Accused of Patent Infringement - McLane Middleton — Willful infringement risks and enhanced damages exposure
34. Part 25 – Interim Remedies and Security for Costs - Civil Procedure Rules — UK civil procedure rules governing security for costs orders
35. Security for costs—claimant resident outside the jurisdiction - LexisNexis — Grounds for security for costs orders against foreign parties
36. Security for costs—what is it, its use and the court's discretion - LexisNexis — Overview of security for costs in UK litigation and court's discretionary powers
37. Dealing with a cease and desist letter: part 1 - Lexology — Importance of timely and appropriate response to cease and desist letters
38. Patent Litigation: Patent Settlement Agreements - Practical Law — Pre-suit settlement considerations and strategic benefits
39. Patent Litigation Pre-Suit Toolkit - Practical Law — Pre-litigation considerations and settlement options
40. 7 Key Takeaways For Litigating Willful Patent Infringement - Mondaq — Court consideration of willfulness and good faith defenses
41. 265103%2Fmedia%2Fslspublic%2FChristopher+B.+Seaman+-+Willful+Patent+Infringement+and+Enhanced+Damages+After+In+re+Seagate.pdf">Willful Patent Infringement and Enhanced Damages After In re Seagate - Stanford Law — Empirical analysis of factors courts weigh in willfulness determinations
42. So, your company just received a cease and desist letter - Lexology — Strategic importance of assessment and evidence gathering before responding
43. An International Guide to Patent Case Management for Judges - United States - WIPO — Standards for preliminary injunctions in patent cases
44. Interim injunctions—the American Cyanamid guidelines - LexisNexis — UK test for granting interim injunctions
45. An International Guide to Patent Case Management for Judges - United Kingdom - WIPO — UK interim injunctions in patent proceedings and special considerations
46. Preliminary Injunctions: Irreparable Harm and the Role of the Damages Expert - American Bar Association — US preliminary injunction four-factor test and irreparable harm analysis
47. Revisiting eBay: A Can of Worms - American Bar Association — eBay standard eliminating automatic presumption of irreparable harm in patent cases
48. Rex Medical v. Intuitive Surgical - CAFC — Case example showing continued business operations during patent litigation
49. Intellectual Property Liability Insurance - Aon — Overview of IP insurance for patent infringement defence
50. Intellectual property insurance - UK Government — Official UK guidance on IP insurance types and coverage options
51. Intellectual property insurance - CFC — IP insurance coverage types including defence and enforcement
52. Patent litigation funding and insurance - D Young & Co — Before-the-event and after-the-event insurance options for patent litigation
53. Defence Insurance - IPISC — Cost savings data showing 50-75% reduction in patent defence costs with insurance
54. Defence Insurance For Intellectual Property Risks - IPISC — Average patent defence costs and insurance coverage benefits