According to the Enlarged Board in its opinion G 10/91 (OJ 1993, 420; see also G 9/91, OJ 1993, 408), an opposition division or a board of appeal is not obliged to consider all the grounds for opposition referred to in Art. 100 EPC, going beyond the grounds covered by the statement under R. 55(c) EPC 1973 (R. 76(2)(c) EPC), but may exceptionally do so. In opposition proceedings, the opposition division may, in application of Art. 114(1) EPC, raise a ground for opposition not covered by the opposition statement or consider such a ground raised by the opponent (or referred to by a third party under Art. 115 EPC) after the expiry of the time limit of Art. 99(1) EPC in cases where, prima facie, there are clear reasons to believe that it is relevant and would in whole or in part prejudice the maintenance of the European patent.
However, in opposition appeal proceedings a more restrictive application of Art. 114(1) EPC is generally justified. A fresh ground for opposition should only be raised by the board of appeal or admitted into the proceedings if the board considers it already prima facie highly relevant and the patentee agrees to its introduction (G 10/91). For details see chapter V.A.3.2.1 h) "Fresh ground for opposition on appeal".
The wording of Art. 101(1) EPC ("at least one ground for opposition") now makes it clear that the opposition division is not obliged to examine all the grounds for opposition. Reference is also made to this chapter IV.C.3.4. "Fresh grounds for opposition".
Following G 10/91, the board in T 274/95 (OJ 1997, 99) held that, if a ground for opposition was substantiated in the notice of opposition but was subsequently not maintained during the opposition division proceedings (in the case in question, a statement to that effect had been made by the opponent during oral proceedings), the opposition division was under no obligation to consider this ground further or to deal with it in its decision, unless the ground was sufficiently relevant to be likely to prejudice maintenance of the patent.
In T 223/95 the appellant had argued that the opposition division should take steps of its own motion to establish by investigation the level of knowledge of the person skilled in the art. The board observed that such an investigative approach would not be consistent with the character of the post‑grant opposition proceedings under the EPC, which were in principle to be considered as contentious proceedings between parties normally representing opposite interests, who should be given equally fair treatment. In the board's opinion, it was unrealistic to suppose that the opposition division could seek, gather and select evidence in an entirely impartial fashion; herein lay an essential distinction between the functions of the examining divisions and the opposition divisions. It was the responsibility of the opponent himself to present to the opposition division the facts, evidence and arguments in support of the grounds on which the opposition was based.