Under R. 89 EPC, intervention is not possible until the opposition period has expired (this was previously laid down in Art. 105 EPC 1973), but notice of it must be filed within three months of the date on which proceedings referred to in Art. 105 EPC were instituted.

In T 452/05 the board considered three dates which could possibly have triggered the three month period: the date on which the request for an interim injunction was made, the date on which the request was granted, or the date when the injunction order was served upon the opponent. In the board's view, only the last date should be regarded as the decisive point in time, as only from that date onwards could the opponent provide evidence of the proceedings that entitled it to intervene.

If multiple proceedings are under way, the three-month time limit for intervening is always triggered by the date on which the first proceedings were instituted (T 296/93, T 1143/00).

The two alternative starting points under Art. 105(1) EPC 1973 for calculating the three-month period for intervention were mutually exclusive (T 296/93, OJ 1995, 627). See also T 144/95. Art. 105 EPC 1973 could not be used to give an opponent who failed to file an appeal in time a second chance, as the opponent does not fulfil the requirement of being a third party (T 1038/00).