The admissibility of the opposition must be checked ex officio in every phase of the opposition and ensuing appeal proceedings. It can and, where appropriate, must be raised by the board in appeal proceedings even if this is the first time this matter is addressed (T 289/91, OJ 1994, 649). On the other hand, the requirement of admissibility must not be circumvented by overemphasizing the ex officio principle and shifting the burden of establishing the case from the opponent to the opposition division. In this context, the substantiation of the opposition is not a requirement per se, but has a clear purpose: that the opposition division can clearly set out the case just on the points at issue. The (minimum) requirements for substantiation of the opposition have their counter-part in the (minimum) requirements for the reasons to be given in the decision of the opposition division. There is insufficient reasoning of a decision if the "when, where and what" is not set out in detail, leaving the reader to guess what the facts underlying the decision are, why and on what evidence they were based, and how the evidence was evaluated (cf. T 522/94, OJ 1998, 421).
The legal basis for filing an opposition is Art. 100 EPC, which contains an exhaustive list of grounds. Accordingly, an opposition may only be filed on the grounds that the subject-matter of the European patent is not patentable, the European patent does not sufficiently disclose the invention or the subject-matter of the European patent goes beyond what was originally disclosed. Each ground for opposition listed in Art. 100 EPC corresponds to a requirement for grant laid down elsewhere in the EPC.
The Enlarged Board of Appeal has set out a number of fundamental principles in G 1/95 (OJ 1996, 615):
Whereas the grounds for opposition in Art. 100(b) and (c) EPC 1973 each relate to a single, separate and clearly delimited legal basis on which an opposition could be based, i.e. insufficient disclosure or unallowable amendment before grant, the same does not apply to Art. 100(a) EPC 1973. Art. 100(a) EPC 1973 does not only refer to the general definition of patentable inventions under Art. 52(1) EPC 1973 and the exceptions to patentability under Art. 53 EPC 1973; it also refers to a number of definitions given in Art. 52(2) to (4) EPC 1973 and Art. 54 to 57 EPC 1973 for the terms "invention", "novelty", "inventive step" and "industrial application", which, when used in conjunction with Art. 52(1) EPC 1973, set specific requirements and therefore constitute separate grounds for opposition. Taken together, Art. 52 to 57 EPC 1973 therefore do not, for the purposes of Art. 100(a) EPC 1973, constitute a single objection to the maintenance of the patent, but a collection of different objections. Some of these are completely independent of each other (e.g. Art. 53 EPC 1973 and Art. 52(1) EPC 1973, Art. 54 EPC 1973) whereas others might be more closely related (e.g. Art. 52(1) EPC 1973 and Art. 54 EPC 1973? EPC; Art. 52(1) EPC 1973 and Art.56 EPC 1973). For an opposition to be admissible under Art. 100(a) EPC 1973, it has to be based on at least one of the grounds for opposition set out in Art. 52 to 57 EPC 1973.
One function of R. 76 and 77 EPC (former R. 55 and 56 EPC 1973) is to establish what the notice of opposition has to contain in order to be admissible. R. 76(2)(c) EPC specifies that the notice of opposition has to contain a statement of the extent to which the European patent was opposed and of the grounds (i.e. the legal reasons mentioned above) on which the opposition is based and an indication of the facts, evidence and arguments presented in support of these grounds, i.e. the substantiation. The wording of (c) shows clearly the distinction made between the grounds, meaning the legal reasons or legal bases such as those under Art. 100(a) EPC and the substantiation. Accordingly, in the context of Art. 99 and 100 EPC and of R. 76(2)(c) EPC, a "ground for opposition" has to be interpreted as meaning an individual legal basis for an objection to the maintenance of a patent. It followed in particular that Art. 100(a) EPC contained a collection of different legal objections (i.e. legal bases), or different grounds for opposition, and was not directed to a single ground for opposition.
In T 600/08 the applicant named in the divisional application at the time of filing was not identical to the applicant which had filed the parent application. The latter had thus submitted a request under R. 139 EPC (former R. 88 EPC 1973) for the name of the applicant to be corrected - which was granted by the receiving section. The appellant/opponent argued in the opposition proceedings that the receiving section had been wrong to grant this request for correction. The respondent/patent proprietor had countered that it was not admissible to raise an objection of this nature in opposition proceedings. According to the board, the grant of a patent remedied any formal errors or deficiencies arising during the grant procedure, including any unlawful grant by the receiving section of a request for correction under R. 139 EPC (former R. 88 EPC 1973). Thus, in the absence of valid grounds for opposition under the EPC, the patent granted could not be revoked on such a basis in opposition proceedings. Moreover, the fact that the text of the contested patent had been changed was of no consequence; the "cut-off" effects associated with the patent grant applied to any formal errors or deficiencies which had arisen during the grant procedure in this respect too.