Warning letters as marketing – when legal correspondence becomes unlawful

What was the case about?

A company sent two warning letters to a competitor and claimed that the competitor's product infringed a European patent which, according to the letter, was validated in Sweden amongst other countries. It transpired, however, that the information about the patent's validation in Sweden was incorrect.

The case illustrates a problem that many companies face: how does one draft a strongly worded warning letter that achieves the desired result, whilst avoiding the correspondence being classified as unlawful marketing?

When does a warning letter become marketing?

According to the Marketing Act, marketing is defined as advertising and other measures in commercial activity that are liable to promote the sale of or access to products. The concept of marketing is extremely limited by the requirement that the measures must be of a distinctly commercial character – that is, undertaken in commercial activity with the purpose of promoting sales.

The Patent and Market Court of Appeal has, in an earlier case from 2023, PMT 13795–22, emphasised that warning letters are often an entirely necessary element in a serious dispute. Warning letters should therefore, as a rule, not be considered to constitute marketing if they contain factual and nuanced information about a party's position in a dispute.

In the present case, however, the court made a different assessment and found that the warning letters constituted unlawful marketing.

The sender of the warning letters has appealed the judgment to the Patent and Market Court of Appeal, which in September 2025 granted leave to appeal.

What made the warning letters problematic?

The court identified several critical deficiencies in the warning letters which meant that they were assessed to constitute marketing:

• Assertions about the patent's validation in Sweden were not substantiated in a satisfactory manner

• Findings of infringement were presented without further analysis or argumentation

• Far-reaching demands for cessation of commercial activity without sufficient grounds

The court found that the warning letters as a whole conveyed conclusions and assertions rather than arguments and analyses regarding the infringement issue. The letters did not signal that the assessments were the sender's own and that there could be scope for the counterparty to hold different positions. Instead, the message was presented as objective findings of already established infringements that there was no scope to question.

The significance of incorrect information in the letters

According to the Patent and Market Court's assessment, incorrect information in warning letters can reinforce the lack of objectivity in the correspondence and weakens the impression that the letters constitute serious legal correspondence. It can also strengthen the perception that the letters have a sales-promoting purpose.

Whether the incorrect information was included in the letters deliberately or by mistake need not, according to the court's assessment, affect the evaluation. This means that even unintentional errors can be significant for the assessment of whether a warning letter constitutes marketing.

Practical advice for drafting warning letters

Based on the judgment, we recommend the following:

• Ensure factual accuracy: Ensure that all information is correct and carefully verify all details about patent validation, registrations and other rights before the letter is sent. Incorrect assertions can affect the assessment even if they are unintentional.

• Avoid categorical assertions: Be careful about presenting conclusions as objective facts. Use formulations that make it clear that it is your assessment, for example "in our view", "based on our analysis" or "as far as we have been able to ascertain".

• Include analysis and argumentation: Do not merely present conclusions but also the analysis and arguments that form the basis for your claims. This strengthens the impression of serious legal correspondence and reduces the risk of the letter being perceived as marketing.

• Nuance the demands: Avoid overly far-reaching and categorical demands. Formulate demands in a way that opens up for dialogue and negotiation.

• Document and analyse: Before you send a warning letter, ensure that you have a thorough analysis of both the rights issue and the infringement issue. This not only facilitates the drafting of the letter but also provides a better foundation if the dispute should escalate.

Summary

The judgment from the Patent and Market Court shows that the boundary between legitimate legal correspondence and unlawful marketing can be difficult to determine. Although warning letters are a natural and accepted procedure in civil law disputes, they can under certain circumstances be classified as marketing – particularly if they contain incorrect information, categorical assertions without substantiation, or appear overly one-sided and far-reaching.

For companies that need to send warning letters, this means that such letters must be drafted with great care. Focus on objectivity, nuance and correctness. By following these principles, you can both achieve the desired result with the warning letter and avoid the risk of the correspondence being classified as unlawful marketing.

Do you have questions about how best to draft warning letters, or wish to discuss the implications of this ruling for your business? Please do not hesitate to get in touch.