Under Art. 105 EPC, third parties may intervene only if they prove either that proceedings for infringement of the opposed European patent have been instituted against them or that, following a request by that patent's proprietor that they cease the alleged infringement, they have instituted proceedings for a ruling that they are not infringing the patent (G 4/91, OJ 1993, 707 in conjunction with Art. 105 EPC). The principle that the assessment of the nature of a procedural act taken by a party before the EPO is to be made with consideration of its actual substance rather than with consideration of its form or name applies equally when deciding whether national infringement proceedings have been commenced (T 188/97).

In T 195/93 the board held that if a patentee simply sent a legal warning letter asking the intervener to stop production and sale of a product covered by the patent, this could not be regarded as institution of proceedings for a court ruling even if under national law the addressee became liable for damages 30 days after receiving the letter. The intervention was therefore rejected as inadmissible.

Similarly, according to T 392/97, Art. 105 EPC 1973 required that an intervener should prove both that the patentee had requested that he cease alleged infringement of the patent and that he had instituted proceedings for a court ruling that he was not infringing the patent. Thus where the letter was sent not to the allegedly infringing company, but to an unrelated one which it was assumed was about to acquire it, the first requirement was not fulfilled and their interventions were inadmissible. The first requirement was also not fulfilled in T 446/95, where the letter merely set out a position adopted in response to a letter from the purported intervener, and did not give notice to cease infringement. According to T 887/04 of 14 November 2006 date: 2006-11-14, it was for the assumed infringer to prove, first, that he had received a "request" to cease the infringement, i.e. that he had been expressly asked to do so, and, second, that he had been required to "cease", i.e. put an end to, the infringement. Proof of mere warnings or threats of legal action had therefore to be considered insufficient.

In T 898/07 the board found the intervention admissible. It was apparent from the documents submitted by the opponent that the respondent had repeatedly informed it that it considered its actions to infringe the patent and, with reference to infringement proceedings already pending against third parties, invited it to enter licensing negotiations. The board considered this a request to cease patent infringement by concluding a licensing agreement which would render the allegedly unlawful conduct (patent infringement) lawful (licensed use). This met the first requirement under Art. 105(1)(b) EPC, the wording of which related not only to a request to refrain from using the invention altogether, but also generally to a request to cease the alleged patent infringement. In addition, the opponent had instituted proceedings against the respondent for a declaration of non-infringement of the patent.

In T 223/11 the board held that it is internationally a widely accepted principle that the submission of a request on the national level for a marketing authorisation for a pharmaceutical product by a generic company does not constitute patent infringement (the so-called Bolar exemption). Such proceedings, no matter how obstructive these may have been to a future market entry of the intervener's product, cannot be considered as equivalent to infringement proceedings, irrespective of whether the Bolar exemption has been codified in the relevant national law or not.

In T 1713/11 the board stated that the EPC gives no specific definition of what constitutes proceedings for infringement. Furthermore, intervention was conceived as a procedurally exceptional situation, which is justified only by a substantial legitimate interest of the assumed infringer to enter the opposition proceedings. Therefore, it did not appear likely that there was a legislative intent to create an elaborate and intricate system of procedural provisions for the admissibility of interventions, and the boards too should refrain from creating such a system through their case law, and focus instead on whether the substantive conditions for admitting an intervention were fulfilled. Nullity proceedings concerning the patent at issue would not qualify and neither would saisie-contrefaçon proceedings (see also T 305/08). However, the definition of "proceedings for infringement" was not limited to civil proceedings, to proceedings that allow for certain remedies to be claimed, or two party proceedings. The board therefore concluded that as long as a patent proprietor or any other party entitled to do so initiated proceedings which aimed to establish whether a third party was commercially active in an area that fell within the patent proprietor's right to exclude, such proceedings were "proceedings for infringement" within the meaning of Art. 105 EPC.

In T 1746/15 the board followed T 1713/11, according to which "proceedings for infringement" pursuant to Art. 105(1)(a) EPC were "proceedings meant to establish whether a third party is commercially active in an area that falls within the patentee's right to exclude". The board held that the independent evidentiary proceedings under German law ("selbständiges Beweisverfahren") initiated against the respondent by decision of the Landgericht Düsseldorf do not constitute the "establishment" of an infringement within the above meaning. The independent evidentiary proceedings under German law is comparable to "saisie" proceedings, e.g. "saisie-contrefaçon" in France, which are not regarded as "infringement proceedings" in terms of Art. 105(1)(a) EPC (T 1713/11 and T 305/08).