According to Art. 99(1) EPC, within nine months of the publication of the mention of grant of the European patent any person may give notice to the EPO of opposition to that patent. Opposition proceedings (see Art. 99 EPC to Art. 105 EPC and R. 75 EPC to R. 89 EPC) constitute an exception to the general rule under the EPC that after grant a European patent is no longer within the competence of the EPO but becomes a bundle of patents within the jurisdiction of the designated contracting states (T 9/87, OJ 1989, 438). They allow, during a limited period of time, a centralised action for revocation of a European patent to be brought before and decided by the EPO.
Opposition is an independent procedure following the grant procedure (T 198/88, OJ 1991, 254). It is not designed to be an extension of the examination procedure (G 1/84, OJ 1985, 299). As explained in G 9/91 and G 10/91 (OJ 1993, 408 and 420), it takes place at a point in time when the proprietor is enjoying in each designated contracting state the same rights as would be conferred by a national patent granted in that state (Art. 64 EPC). The relief sought by the opponent is not, as in traditional pre-grant opposition, refusal of the patent application but revocation of the patent as granted (in its entirety or in part) with effect ex tunc in all designated contracting states (Art. 68 EPC). Furthermore, the grounds for opposition (Art. 100 EPC) being limited to and essentially the same as the grounds for revocation under national law (Art. 138 EPC), the concept of post-grant opposition under the EPC differs considerably from that of pre-grant opposition. The opposition procedure before the EPO thus has several important features more in common with the concept of the traditional revocation procedure and its effect is more similar to that of such a procedure. This characteristic is further emphasised by the addition at a late stage in the preparation of the EPC of the possibility to oppose a European patent even if it has been surrendered or has lapsed for all contracting states (Art. 99(3) EPC 1973; see R. 75 EPC and also T 606/10). As opposition is an independent procedure which takes place after the grant procedure, a document considered in examination proceedings is not automatically scrutinised in opposition or opposition appeal proceedings, even if quoted and acknowledged in the contested European patent (T 198/88, OJ 1991, 254).
The opposition procedure is a purely administrative procedure. It differs fundamentally from the appeal procedure, which must be regarded as a judicial procedure and is, by its very nature, less investigative (see G 7/91 and G 8/91, OJ 1993, 356 and 346; G 9/91, OJ 1993, 408). Unlike an appeal, opposition has neither suspensive effect nor the effect of transferring the case to a superior instance (see T 695/89, OJ 1993, 152).
Post-grant opposition proceedings under the EPC are in principle to be considered as contentious proceedings between parties who normally represent opposing interests and who should be treated equally fairly (G 9/91 and G 10/91, OJ 1993, 408 and 420). As soon as the notice of opposition has been filed, the procedure automatically becomes bilateral, no matter whether the opposition is valid, admissible or allowable. The Convention does not provide for a decision by an opposition division "to end the ex parte proceedings" (T 263/00). R. 79(1) EPC (R. 57(1) EPC 1973) requires the opposition division to communicate the opposition to the proprietor of the patent, and the Guidelines D‑IV, 1.5 (March 2022 version) provide that communications and decisions about whether the opposition is deemed to have been filed and is admissible are also to be notified to the patent proprietor. Concerning the contentious nature of opposition proceedings, see also parallel cases T 1553/06 and T 2/09 in this chapter IV.C.2.1.4.
According to the principle of party disposition (cf. G 9/92 date: 1994-07-14 and G 4/93, OJ 1994, 875), the request of the opponent determines the extent to which the patented subject-matter may be examined in opposition. In T 364/18, for instance, the board saw itself limited to examining the patent with regard to the claims opposed, in accordance with the principle of ne ultra petita (on the extent of opposition, see also chapters IV.C.2.2.6 and IV.C.3.2).
Opposition proceedings are characterised by the interplay between its nature as contentious proceedings between parties (G 9/91 and G 10/91, OJ 1993, 408 and 420) and the principle of ex officio examination which applies to proceedings before the EPO (Art. 114(1) EPC) and thus also in opposition proceedings (G 9/92 date: 1994-07-14 and G 4/93, OJ 1994, 875). Accordingly, in opposition proceedings, the EPO must investigate the facts of its own motion (Art. 114(1) EPC). However, certain restrictions have been placed on this principle in order to make opposition proceedings clearer, to shorten and accelerate cases, and to limit the risks to the parties (G 9/92 date: 1994-07-14 and G 4/93, OJ 1994, 875). In particular, ex officio examination has a restricted application with respect to the grounds for opposition: grounds extending beyond those properly covered by the opponent's statement pursuant to R. 76(2)(c) EPC are only exceptionally considered (G 9/91 and G 10/91; for details, see chapter IV.C.3.3 and 3.4).
Under the EPC, there is no legal obligation for the patent proprietor to take an active part in opposition proceedings (e.g. in reply to the communications under R. 81(3) EPC or R. 116(2) EPC). However, the patent proprietor is not free to present or complete his case at any time that he wishes during the opposition or opposition appeal proceedings (see e.g. T 936/09 and T 167/11; see also the case law in chapters IV.C. 5.1.4 "Timeframe for filing amendments", V.A.4.3.4 "Discretion under Article 12(4) RPBA 2020", V.A.4.3.7 "Submissions that should have been submitted at first instance – Article 12(6), second sentence, RPBA 2020".
Certain topics relating to opposition proceedings are dealt with in Part III: See chapters III.O. "Transfer of party status", III.P. "Intervention", III.Q. "Continuation of the opposition proceedings by the EPO" and III.R. "Apportionment of costs".