1. Introduction

According to Art. 105 EPC, any third party may, in accordance with the Implementing Regulations, intervene in opposition proceedings after the opposition period has expired, if the third party proves that proceedings for infringement of the same patent have been instituted against them, or following a request of the proprietor of the patent to cease alleged infringement, the third party has instituted proceedings for a ruling that they are not infringing the patent. Under Art. 105(2) EPC an admissible intervention shall be treated as an opposition.

The legal concept of intervention, as set out in Art. 105 EPC, establishes a special legal framework for the institution of an opposition outside the period for filing an opposition provided for in Art. 99 EPC, but still before the competent departments of the EPO, which include the boards of appeal. This exceptional nature inherently precludes an extensive interpretation and application of this legal remedy (G 2/24, OJ 2026, A24). Intervention is provided for when both opposition proceedings before the EPO and infringement proceedings before a national court are in existence at the same time, in respect of the same European patent (G 4/91, OJ 1993, 707).

Article 105 EPC is intended to provide third parties, after the expiry of the opposition period, with the right to have the validity of a European patent, which they are alleged to infringe, examined in the centralised procedure provided for under the EPC at the administrative stage before the opposition division and/or at the judicial stage before the boards of appeal (G 1/94, OJ 1994, 787, G 2/24, OJ 2026, A24).