Patent warning letter

A patent warning letter is the letter by which the patent holder informs a third party of the existence of its patent and of the infringement it considers to have occurred. No provision requires one before proceedings are issued, but Article 54 of the Code of Civil Procedure requires the originating document to mention the steps taken towards an amicable resolution of the dispute, which explains why such letters are frequently sent.

Legal effects of the warning

The warning ends the recipient’s good faith. Article L. 615-1, paragraph 3, of the Intellectual Property Code makes the liability of a person who offers, puts on the market, uses or holds an infringing product, without being its manufacturer, conditional on knowledge. The letter creates that knowledge and opens the period for which compensation may be claimed, as explained in the entry on good faith in infringement. It also fixes, as regards the patent holder itself, a date of knowledge of the facts that can be relied on for limitation purposes.

Risks of a warning letter

Sent to the competitor’s customers or distributors, drafted in peremptory terms or circulated publicly, the letter exposes its author to an unfair competition action for disparagement, under Article 1240 of the Civil Code. Asserting an infringement that no decision has yet established is the textbook case. The letter also reveals the patent relied on and gives the recipient time to organise its defence, through an invalidity action, an action for a declaration of non-infringement or the filing of a protective letter with the Unified Patent Court.

Drafting

A useful warning letter identifies the patent and the claim relied on, describes the product concerned, sets out the comparison and makes a precise request with a deadline. It remains measured in its terms and targets only the operator concerned. The tone, the recipient and the circulation of the letter matter as much as its legal content.

Key points

The letter triggers effects that cannot be undone: it opens the period for which compensation may be claimed, but it also forfeits the element of surprise and may date the patent holder’s knowledge of the facts. The decision to send it is therefore taken after the seizure and after the validity analysis, not before.

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This glossary entry is general in scope and does not constitute legal advice.

Dhenne Avocats litigates patent disputes before the Paris Judicial Court, the Paris Court of Appeal and the Unified Patent Court. Where the question arises in a real case, the useful moment is almost always before proceedings are issued.

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