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 he 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. See also 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.