2.4.1 Intervention in opposition proceedings

In G 4/91 (OJ 1993, 707) the Enlarged Board held that the intervention under Art. 105 EPC 1973 of an assumed infringer in the opposition proceedings presupposed that opposition proceedings were pending at the time they gave notice of intervention. Moreover, a decision of the opposition division on the relief sought had to be regarded as conclusive in the sense that the opposition division no longer had the power thereafter to amend its decision. The Enlarged Board further held that proceedings before an opposition division were terminated when such a conclusive decision was issued, irrespective of when this decision became final. Thus, if, after an opposition division had issued a decision terminating the proceedings, and none of the parties to the opposition proceedings filed an appeal, any notice of intervention filed during the two-month period for appeal under Art. 108 EPC 1973 was invalidated.

In T 791/06, where an intervention was filed after the oral proceedings before the opposition division when the decision was pronounced but before notification of the written decision, the board held that it had not been filed during pending opposition proceedings but considered the intervention to have been filed during the appeal proceedings.

In T 631/94 (OJ 1996, 67) the board noted that when a decision to terminate opposition proceedings, taken in written proceedings, was handed over to the EPO postal service, it became public and effective and had therefore been issued. If the parties to the proceedings leading to that decision did not appeal, the opposition proceedings were completed at that point in time and thereafter intervention based on Art. 105 EPC 1973 was no longer possible.

In T 2328/22 the board upheld the opposition division's decision that that the scope of the intervention of opponent II (appellant) was limited to objections concerning the adaptation of the description (res judicata, Art. 111(2) EPC). Article 105(1) EPC allowed interventions only in pending opposition proceedings and precluded interventions from opening entirely new proceedings or an independent stage in proceedings. This interpretation was also consistent with the finding in G 4/91 that an intervention had no legal effect if it was filed after the opposition division's decision had been issued. As the opposition proceedings were pending only with respect to the adaptation of the description, the intervener was no longer entitled to intervene in the procedurally concluded part of the opposition proceedings which concerned the validity of the wording of the claims.