1.
Legal character of opposition proceedings and applicable principles 

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 to 105 and R. 75 to 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. 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 now R. 75 EPC and also T 606/10).

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 (cf. 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 (November 2015 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.D.2.1.4.

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).

According to the principle of party disposition (cf. G 2/92 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 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 of ex officio examination in order to make such proceedings clearer, to shorten and accelerate cases, and to limit the risks to the parties (G 2/92 and G 4/93, OJ 1994, 875).

Under the EPC, there is no legal obligation for the patent proprietor to take an active part in opposition proceedings. 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 (T 936/09 – see also summary in Chapter IV.E.4.3.2c) and decision T 167/11).

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).

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