An objection that an opposition is inadmissible because the opponent was not entitled to file it may be raised at any stage of the proceedings, i.e. even at a late stage before the board of appeal, because the admissibility of the opposition is an indispensable procedural requirement for any substantive examination of the opposition submissions and is therefore to be examined ex officio (see T 289/91, OJ 1994, 649; T 590/94, T 522/94, OJ 1998, 421; T 1178/04, T 384/08). Following the principle established by board of appeal case law that the admissibility of an opposition is examined by the EPO of its own motion, if the board has good reasons for examining the admissibility of an opposition (T 199/92), it may and is obliged to do so, even if the patent proprietor has not challenged admissibility during either opposition or appeal proceedings (T 541/92). In G 3/97 and G 4/97 (OJ 1999, 245 and 270), the Enlarged Board of Appeal reaffirmed that the admissibility of an opposition could be challenged during the appeal proceedings on grounds relating to the identity of an opponent, even if no such objection had been raised before the opposition division.

During the appeal underlying G 1/13 (OJ 2015, A42), the proprietor had challenged the admissibility of the opposition because the opponent company had ceased to exist during the proceedings before the opposition division. By court order, the company was then restored with retrospective effect. The Enlarged Board held that where an opposition is filed by a company which subsequently, under the relevant national law, ceases to exist, but is subsequently restored to existence under a provision of that national law, by virtue of which the company is deemed to have continued in existence as if it had not ceased to exist, all these events taking place before a decision of the opposition division maintaining the opposed patent in amended form becomes final, the EPO must recognise the retroactive effect of that provision of national law and allow the opposition proceedings to be continued by the restored company (see also chapter V.A.2.4.3 d) "Existence of a company").

For decisions concerning transfer of opponent status, see chapter III.O.2.

The status of opponent can also be acquired by an assumed infringer who has intervened in pending opposition proceedings (see chapter III.P. "Intervention").