Opposition proceedings are not a continuation of examination proceedings. Hence as a general rule the opposition division will confine its examination to those grounds for opposition brought forward by the opponent. If, for example, the opposition is filed only on the grounds that the subject-matter of the European patent is not sufficiently disclosed or that it extends beyond the content of the patent application as filed, the opposition division will examine the patentability of the subject-matter of the European patent pursuant to Art. 52 to Art. 57 only if facts have come to its notice which, prima facie, wholly or partially prejudice the maintenance of the patent (see G 10/91).
A document indicated in the patent specification as the closest or important prior art for the purposes of elucidating the technical problem set out in the description forms part of the opposition proceedings even if not expressly cited within the opposition period. The same applies to any relevant documents cited in the patent specification which do not constitute the closest prior art but whose contents are nevertheless important for understanding the problem underlying the invention within the meaning of Rule 42(1)(c) EPC (T 536/88, in particular point 2.1).
Once proceedings for examining the opposition(s) have been initiated because an admissible opposition has been filed (although it may have been withdrawn in the interim), there may be reason to believe that other grounds exist which, prima facie, in whole or in part prejudice the maintenance of the European patent. If that is the case, these grounds should generally be examined by the opposition division of its own motion pursuant to Rule 81(1). Such other grounds may result from facts emerging from the search report or the examination procedure, the examiner's personal knowledge or observations presented by third parties pursuant to Art. 115 (see also E‑VI, 3). Such grounds may also have been put forward in another opposition which has been rejected as inadmissible, or in another opposition deemed not to have been filed. They may also be any grounds submitted belatedly (see E‑VI, 1.1 and E‑VI, 2). Under Art. 114(1), such prejudicial grounds put forward in an opposition which has been withdrawn should also generally be examined by the opposition division of its own motion. In carrying out such examination the opposition division should, however, take the interests of procedural expediency into account (see E‑VI, 1.2). If the decision is to be based on grounds to be taken into account pursuant to Art. 114(1) or Rule 81(1), the parties must be given the opportunity to comment (see E‑X, 1).
If during examination of the opposition an allegation about a relevant fact seems plausible, it may be taken into account without further evidence if it is not challenged by the other party.
If a fact is contested or not plausible, the party making the allegation must prove it. If the parties to opposition proceedings make contrary assertions which they cannot substantiate and the opposition division is unable to establish the facts of its own motion, the patent proprietor is given the benefit of the doubt (see T 219/83, Headnote I).
For example, if the opponent raises an objection under Art. 100(b) and provides experimental evidence that e.g. the claimed process cannot be realised, and the patent proprietor replies that the process can be carried out without undue burden by the skilled person taking common general knowledge also into consideration (T 281/86, OJ EPO 1989, 202; reasons 6), the patent proprietor has to provide proof of what was common general knowledge at the filing date (or the date of the earliest priority if priority has been claimed).
Since unity of invention under Art. 82 is only required for the European patent application, the unity of the subject-matter of the European patent may not be examined by the opposition division, even of its own motion. In particular, where the facts, evidence and arguments which come to light in the opposition proceedings lead to the maintenance of the European patent in amended form, there should be no further examination as to whether the remaining subject-matter of the patent contains a single invention or more than one. Any lack of unity must be accepted (see G 1/91).
The grounds for opposition laid down in Art. 100 are examined in greater detail below.