Strict Diligence Required for Case Amendments under Rule 263

Court of Appeal upholds strict diligence requirement for case amendments under R. 263 RoP – late filed prior art not admitted into the proceedings

Case: CeraCon v. Sunstar — Court of Appeal, 2 Sept 2025 (UPC_CoA_807/2025)

The Court of Appeal has dismissed CeraCon’s request for discretionary review of an order of the Mannheim Local Division refusing leave to amend a counterclaim for revocation. The case turns on the strict diligence requirement of R. 263.2(a) RoP: if a prior art attack could have been raised earlier with reasonable effort, it cannot be added later.

Background

The underlying infringement action was lodged by Sunstar before the Mannheim Local Division in November 2024. CeraCon filed a counterclaim for revocation in March 2025. Two months later, CeraCon sought leave to amend its case by adding a novelty attack based on Euro-PCT application EP 3 868 480 (“EP’480”).

The judge-rapporteur refused leave, holding that CeraCon had not shown reasonable diligence in its prior art search. A subsequent application under R. 333 RoP was rejected by a panel of the Local Division. The Mannheim LD also refused leave to appeal. CeraCon therefore turned to the Court of Appeal, requesting a discretionary review under R. 220.3 RoP.

The Court’s reasoning

The Court of Appeal recalled that R. 263 RoP permits amendments only if two cumulative conditions are satisfied:

(a) the amendment could not have been made with reasonable diligence at an earlier stage, and

(b) the amendment does not unreasonably hinder the other party.

Failure under either limb is sufficient to bar leave.

Here, CeraCon acknowledged that EP’480 had not been found in its instructed prior art search but was later identified by one of its attorneys using the same search string, where it appeared as the fifth hit. Without convincing explanations as to why this document had been overlooked, CeraCon had not shown that reasonable diligence had been exercised.

The Court concluded that the Local Division had acted within its discretion in refusing leave. There was no manifest error, nor any breach of proportionality, fairness, flexibility, or procedural efficiency.
 

Takeaways

• R. 263.2 RoP sets a high bar: amendments are excluded if either the diligence requirement (a) or the fairness requirement (b) is not met.
• Reasonable diligence in prior art searches is scrutinised closely. If a document is easily retrievable with standard queries, failure to cite it at the outset will generally bar its later introduction.
• The Court of Appeal will not second-guess the Local Division’s discretion absent a manifest error.

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