1.4. Checks for validity of the priority right during examination and opposition proceedings

The EPO does not normally check the validity of a priority right during examination. A check must be made, however, if relevant prior art has been made available to the public within the meaning of Art. 54(2) EPC on or after the priority date claimed and before the date of filing (priority interval) or if the content of the European patent application is totally or partially identical with the content of another European application within the meaning of Art. 54(3) EPC, such other application claiming a priority date within the above-mentioned period (EPC Guidelines F‑VI, 2.1 – April 2025 version). In opposition proceedings this applies where prior art is invoked in connection with a ground for opposition under Art. 100(a) EPC in relation to which the priority date is of decisive importance (EPC Guidelines D‑III, 5 – April 2025 version). Failure by the respective division to check the validity of the priority right in the circumstances described has been held to be a substantial procedural violation (see T 16/89, T 737/95, T 2170/08). On publications during the priority interval, see also G 3/93 (OJ 1995, 18), summarised in chapter II.D.6.1.