Introduction
In decision T 0644/23 (29 September 2025), the EPO Board of Appeal provided important clarification on a recurring procedural issue in opposition and appeal proceedings: whether the division between preamble and characterising portion must be aligned with the closest prior art used in the inventive-step analysis.
The case arose in a procedural context. An opponent argued that an amended auxiliary request should not be admitted because the two-part structure of the claim allegedly failed to reflect the prior art relied upon in the proceedings. In particular, it was asserted that the preamble had to be based on the closest prior art chosen as the starting point for the inventive-step attack. The Board rejected this view and confirmed two principles that are highly relevant for European patent practice.
Is the Two-Part Claim Form Mandatory Under the EPC?
No. First, the Board recalled that the EPC does not impose a general obligation to use the two-part claim format. Rule 43(1) EPC requires a two-part form only where it is “appropriate” (“zweckdienlich”). The Board emphasised that the two-part structure is a means to support clarity and procedural efficiency, not a formal requirement that must be satisfied regardless of the practical consequences. In the case at hand, the Board noted that forcing a different allocation of features between the preamble and the characterising portion would have required a substantial rewriting of the claim. This was considered neither necessary nor appropriate.
Must a Two-Part Claim Be Delimited Over the Closest Prior Art?
No. The delimitation may be made over any sutiable prior art document. In Reasons 3.1.3 and 3.1.4, the Board explained that Rule 43(1)(a) EPC requires the preamble to contain features “known from the prior art”, but it does not require that those features be derived specifically from the closest prior art used in the inventive-step analysis.
The Board confirmed that the EPC requires delimitation only “to the prior art” — and therefore the delimitation may be made over another suitable single prior art document, even if that document is not the closest prior art used as the decisive starting point for inventive step. The Board explicitly referred to earlier case law (T 980/95) supporting this interpretation.
In other words: A two-part claim does not have to be delimited over the closest prior art on file — delimitation may instead be based on another suitable single prior art document.
What is the Practical Impact of T 0644/23 for European Patent Practice?
Decision T 0644/23 provides valuable guidance for applicants and patentees facing procedural objections in opposition or appeal proceedings. It confirms that:
- Two-part claim drafting is required only when it is procedurally useful.
- If a two-part form is used, delimitation may be based on any suitable single prior art disclosure.
- There is no requirement to align the preamble with the closest prior art used in inventive-step reasoning.
In practice, this offers greater flexibility and legal certainty. It confirms that objections based solely on an alleged mismatch between the two-part claim structure and the closest prior art are not well founded. Parties are not required to distort claim drafting or engage in unnecessary rewriting simply to mirror the inventive-step framework adopted by an opponent or a division. Where a different prior art document offers a clearer and more stable basis for the preamble, it may legitimately be used, even if it is not the closest prior art in the substantive analysis.
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