Can modifying a European patent application undermine the validity of your Belgian patent?

Yes, absolutely. Although a Belgian patent is independent of a European patent, the Dutch-speaking Enterprise Court in Brussels ruled in a judgment of September 2, 2025, that radically limiting claims in EPO proceedings, without amending the Belgian patent, can be considered an ‘extrajudicial confession’ that the original Belgian patent is invalid. Passivity or a purely procedural defense is not sufficient in such cases to prevent a declaration of invalidity.

The facts and context

In a recent dispute before the Enterprise Court in Brussels, two players in the chemical sector faced each other: Devan Chemicals (Plaintiff) and Heiq Christal (Defendant)..

The crux of the matter revolved around the validity of a Belgian patent (BE ‘987) held by the Defendant. The situation was as follows:

• Parallel requests: The Defendant had filed both a European and a Belgian patent application for the same invention (a composition for the decontamination of surfaces).
• European resistance: The European Patent Office (EPO) ruled on several occasions that the invention was not new and did not involve any inventive step. In order to save the European patent, the Defendant radically amended its application: from 15 broad claims (mainly product claims) to just 8 limited method claims.
• Belgian standstill: Despite knowing that the original text was considered ‘unpatentable’ in Europe, the Defendant left its Belgian patent unchanged in its original, broad form.

The Plaintiff sought the annulment of the Belgian patent. The Defendant did not raise any substantive defense regarding novelty, but merely requested that the case be postponed (“sent to the roll”) pending the final decision of the EPO.

The court's decision

The court did not go along with the patent holder's delaying tactics and declared the Belgian patent completely invalid. The decision was based on two pillars:

1. No postponement: A Belgian patent is legally independent of the European granting procedure. The court does not have to wait for the EPO, especially when the texts of both patents have become completely different due to the actions of the holder.
2. Extrajudicial confession: The court ruled that the Defendant's conduct constituted an extrajudicial confession. By drastically limiting the claims before the EPO (because the original text was invalid) while at the same time maintaining that same ‘incorrect’ text in Belgium, the Defendant implicitly acknowledged that the Belgian patent in its current form was invalid.

Furthermore, because the Defendant failed to respond substantively to the Plaintiff's technical arguments (the so-called obligation to contest), these arguments were deemed proven by the court.

Legal analysis and interpretation

This ruling contains a number of lessons for IP specialists and patent holders about the interaction between litigation strategy and the law of evidence.

The doctrine of extrajudicial confession

Article 8.1, 10° of the Civil Code defines a confession as the acknowledgment of a fact that may have legal consequences for the person confessing. What is interesting about this judgment is the application of Article 8.31 of the Civil Code: a confession may result from behavior.

The court argues that the objective findings in the EPO file (the file history estoppel concept from Anglo-Saxon law) continue to apply in Belgian proceedings. The fact that the patent holder in Europe withdraws its claims because they are not new creates a presumption of evidence against the validity of the identical Belgian claims.

Obligation to state and obligation to contest

The judgment underscores the importance of the active role of parties in civil proceedings. The plaintiff's burden of proof is counterbalanced by the defendant's obligation to contest the claim (Article 8.3 of the ivil Code and the disposition principle).

The Defendant's strategy of raising only procedural objections (request for postponement) without presenting a substantive defense (e.g., arguing why the invention is indeed new) proved fatal. The court rightly states: “A party cannot refuse to present a substantive defense ‘with reservations.’” In civil cases, silence on the substance of the case amounts to consent.

Independence of procedures

The court confirms the established case law that national and European procedures are independent. The risk of conflicting decisions was non-existent in this case, precisely because the patent holder had allowed the texts to diverge so greatly.. This confirms that Belgian courts are not merely ‘followers’ of the EPO, but exercise their own jurisdiction to the full.

What this means in concrete terms for your patent portfolio

This ruling has direct consequences for companies with a mixed patent portfolio (Belgian + European).

• For the patent holder:
  • Consistency is crucial: You cannot admit to the EPO that your invention is not new (by deleting conclusions) and then pretend nothing happened in Belgium. This will be used against you.
  • Update your Belgian patent: If you need to file with the EPO, consider proactively limiting or amending your Belgian patent through the administrative procedure at the Intellectual Property Office.
  • Always present a substantive defense: Never rely solely on delaying tactics. If the invalidity of your patent is claimed in legal proceedings, you must defend technically why your invention is valid, even if you are primarily requesting a postponement.
• For the party challenging a patent:
  • Use the EOB file: Analyze the correspondence between the opposing party and the EPO. Concessions or limitations there are worth their weight in gold in Belgian proceedings.
  • Force position: If the opposing party remains vague, point out to the judge the violation of the obligation to contest.

Frequently Asked Questions (FAQ)

Must a Belgian court wait for the decision of the European Patent Office?
No. A Belgian request for annulment is separate from the proceedings before the EPO. Although a judge may decide to postpone the case (for example, if there is a risk of conflicting judgments), he is not obliged to do so. Certainly when the patents differ in substance, the proceedings will continue as normal.

What is an extrajudicial confession in patent law?
This is an acknowledgment of a fact outside of legal proceedings. In this context, it means that your conduct in other proceedings (such as drastically limiting your patent at the EPO due to novelty objections) may be seen by the Belgian court as an implicit acknowledgment that your original, broader Belgian patent is invalid.

What happens if I do not defend the substance of my patent in court?
If, as the defendant, you only cite procedural arguments (such as “the court has no jurisdiction” or “the case should be postponed”), but do not explain why your patent is technically valid, you risk the judge considering the plaintiff's arguments to be proven. This will almost automatically lead to the loss of the case and the invalidation of your patent.

Conclusion

Managing intellectual property is more than just submitting forms; it requires strategic coordination between national and international procedures. The Brussels Enterprise Court has made it clear that contradictory behavior will be punished. Allowing a Belgian patent to ‘lie dormant’ in an invalid form while you cease your battle before the EPO makes you vulnerable to annulment and high legal costs.