The filing of an opposition initiates an independent inter partes procedure at the post-grant stage, i.e. 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 and Art. 99 EPC). Thus, 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 (see 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 the pre-grant opposition. The opposition procedure before the EPO thus had several important features more in common with the concept of the traditional revocation procedure and its effect was more similar to that of such a procedure (G 9/91 and G 10/91, OJ 1993, 408 and 420).
Any person having doubts about the legal validity of a European patent may file a reasoned notice of opposition in writing. Opposition is a legal remedy by which granted patents can be revoked or limited but which - unlike an appeal - has neither suspensive effect nor the effect of transferring the case to a superior instance (see T 695/89, OJ 1993, 152). The opponent determines the extent to which the patented subject-matter may be examined in opposition proceedings (principle of party disposition). Therefore, a document taken into account in the procedure before the examining division is not automatically evidence to be considered in opposition or opposition appeal proceedings, even if quoted and acknowledged in the opposed patent (T 198/88, OJ 1991, 254).
The following sections deal in more depth with the content of the fundamental principles applicable to the opposition procedure, which have been addressed in several decisions of both the boards and the Enlarged Board of Appeal.
Following the EPC Revision, the provisions relating to the opposition procedure were largely left unchanged. In particular, Art. 100 and 101 EPC were reworded and, for the sake of clarity, restructured, with part of Art. 102 EPC 1973 being inserted into the new Art. 101 EPC. Certain provisions governing specific aspects of the opposition procedure were moved to the Implementing Regulations. Some of the provisions in the re-numbered R. 75 to 89 EPC were worded more concisely and adapted to the revised EPC version, whilst others were rendered more precise, but none were amended in substance.