When a warning letter arrives from a patent holder: what it costs to be wrong
A warning letter alleging patent infringement starts a clock before any court sees the file. Replying carelessly, admitting facts nobody has checked, or simply waiting can turn a disputable claim into a costly one. The price of being wrong is rarely the underlying claim itself — it is the loss of defences that stay open only for the first few days.
Basis
Where the patent named in the letter is a European patent with unitary effect, the right itself is created under Regulation (EU) No 1257/2012. Its translation regime, which affects how the patent's scope is read in each participating state, sits in Council Regulation (EU) No 1260/2012. Infringement actions over that right go to the forum set up by the Agreement on a Unified Patent Court, not automatically to the recipient's own national court.
Where the dispute concerns goods stopped at an EU border rather than a letter on a desk, a different mechanism applies: Regulation (EU) No 608/2013. That regulation governs how a customs authority holds goods on suspicion of infringement, and it is covered separately in the related situation linked further below.
What changes once the letter lands
The recipient's own position starts moving the moment the letter is opened, whether or not a reply is sent. A forum deciding the dispute later will look at three things: how quickly the recipient acted, what was said in any reply, and whether the disputed activity continued unchanged in the meantime.
None of this is decided by the letter itself. It is decided later, by the forum with jurisdiction over the claim, on the file each side has built up by that point. A reply drafted in haste becomes part of that file. So does silence.
Continuing the disputed activity without a documented view on the patent's validity or scope is treated differently from continuing it after taking advice. That gap is not fixed by a single provision. It is built case by case, from the record each side kept from the day the letter arrived.
The practical cost of getting the response wrong
A weak or premature reply can concede points that were never checked — that the accused product falls within the patent's claims, or that the patent is valid at all. Once conceded, those points are difficult to withdraw later, in any forum.
A record showing continued use after clear notice, with no documented review, is read differently from a record showing a measured pause while the position is checked. The difference does not change the underlying facts of the dispute. It changes how the file looks to whoever decides it.
What to do, in order
- Record the date and channel the letter arrived. The person who received it logs this before forwarding it anywhere else in the organisation.
- Do not reply on the merits before the patent's legal status is checked. An IP-qualified reviewer confirms whether it is granted, still in force, and plausibly covers the accused product or process.
- Identify the forum in advance. Whether the patent is a classic national right or a European patent with unitary effect decides which court or panel would hear the dispute if it escalates.
- Preserve technical records unchanged — design files, test logs, correspondence predating the letter. Assign one custodian and freeze the file as it stood on the date of receipt.
- Route the file to a partner before any deadline in the letter runs out. Put the question to a partner in practice rather than drafting a reply internally first.
- Decide the shape of the reply with counsel. A holding letter, a substantive rebuttal, and silence are three different strategies, not one default response.
Who decides, and on what evidence
The forum depends on the type of right named in the letter, not on where the recipient happens to be based. The table below sets out the three situations this page covers.
| What the letter concerns | Forum | Basis |
|---|---|---|
| European patent with unitary effect | Unified Patent Court | Agreement on a Unified Patent Court |
| Classic national patent, no unitary effect | National court of the state where the alleged infringement occurs | National procedure, not itemised on this page |
| Goods detained at an EU border rather than a letter | Customs authority acting on a request for action | Regulation (EU) No 608/2013 |
Whichever forum applies, the decision turns on the same core evidence: a claim-by-claim comparison of the accused product against the patent's claims, and, where validity is challenged, the prior art each side puts forward. The panel or court decides on the file submitted within the deadlines it sets — not on the assertions made in the original letter.
Three scenarios
- If the letter names a European patent with unitary effect, the Unified Patent Court is the forum, and its own procedural calendar applies from the moment proceedings start. That calendar does not follow any national court's timetable.
- If your own claim to the underlying invention rests on an assignment that was never formally signed, that gap needs resolving before any infringement argument can stand. See When the assignment is unsigned but performed explained.
- If the patent holder also asserts rights through a tokenised licence tied to the same invention, enforcement against that holder follows a different route. See When enforcement is sought against a holder explained.
- If the letter turns out to concern a patent already expired or lapsed for non-payment, the practical question shifts from defence to verifying that lapse before any further correspondence is sent.
What this page does not resolve
This page does not decide whether the patent is valid, whether the accused product actually falls within its claims, or what a court would award if the case proceeded. Those are technical and evidentiary questions, not procedural ones.
Answering them needs a claim chart comparing the product against the patent, a prior-art search if validity is in doubt, and a legal opinion built on that specific file — not a general description of who decides and when.
Where the opposing party is registered outside the EU, the first practical check is often what can be established about that entity before any reply is sent. See Counterparty in United States explained.
Verified
Checked against source texts on 2025-05-01: Regulation (EU) No 1257/2012; Council Regulation (EU) No 1260/2012; Agreement on a Unified Patent Court; Regulation (EU) No 608/2013.
Related situations
A related situation, where the dispute begins with a shipment rather than a letter, is covered in When a customs authority detains a shipment in detail. The starting facts differ, but the same discipline about dates and preserved records applies.
FAQ
What is the first step when a warning letter arrives from a patent holder?
Log the date and channel it arrived, then check the patent's legal status before replying to any claim made in the letter itself.
Which deadline is already running?
Any deadline stated in the letter itself starts running from receipt, whether or not a reply is sent. No other deadline is fixed by the letter alone; the forum sets its own once proceedings start.
Who decides, and on what evidence?
The forum named in the table above decides, on a claim-by-claim comparison of the accused product against the patent and, where raised, on the prior art each side submits within its own deadlines.