A formal requirement for the admissibility of the opposition is the substantiation of the specified ground. Under R. 76(2)(c) EPC (former R. 55(c) EPC 1973) the notice of opposition must contain three items: a statement of (i) the extent to which the European patent is opposed and (ii) of the grounds on which the opposition is based as well as (iii) an indication of the facts, evidence and arguments presented in support of those grounds. For the opposition to be admissible, it is sufficient that one of the grounds for opposition fulfils the requirements of R. 76(2)(c) EPC. Nowhere in the EPC is there any basis for the concept of partial admissibility of oppositions. The concept of "inadmissibility" is only applicable to the notice of opposition as a whole (as held in T 653/99; see also T 212/97 and T 65/00). The boards have already set out the circumstances in which the third requirement of an indication of the facts, evidence and arguments is met. If it is met, the opposition is deemed to be sufficiently "substantiated" and thus admissible.
The question whether a notice of opposition meets the requirements in R. 76(2)(c) EPC must be determined on expiry of the nine-month opposition period. A notice objectively complying with the requirements on the relevant date is not rendered inadmissible where the opponent later cites prior art not mentioned in it to support his line of argument (T 1019/92; see also T 104/06).
According to the board in T 182/89 (OJ 1991, 391), if a ground for opposition is alleged in the notice of opposition but not substantiated as required by R. 76(2)(c) EPC (former R. 55(c) EPC 1973) within the nine-month period for opposition, it had to be rejected on the same basis as if it were inadmissible under R. 77(1) EPC (former R. 56(1) EPC 1973). If this approach was not taken, an opponent could easily abuse the procedure by alleging more than one ground, but supporting only one of them, in the notice of opposition; then, at a later stage in the opposition proceedings, he might bring forward facts and evidence in support of the other alleged grounds for opposition, thus causing delay and an increase in costs. In the board's view, this should not be allowable.
In T 222/85 (OJ 1988, 128) the board of appeal held that the third requirement was only satisfied if the contents of the notice of opposition were sufficient for the opponent's case to be properly understood on an objective basis. The board reasoned that the purpose of the third requirement of R. 76(2)(c) EPC (former R. 55(c) EPC 1973) (in combination with the first two requirements) was to ensure that the notice of opposition set out the opponent's case sufficiently so that both the patentee and the opposition division knew what that case was. Whereas the requirements of R. 76(2)(a) and (b) EPC and the requirements (i) and (ii) of R. 76(2)(c) EPC could be considered as formal in nature, the third requirement of R. 76(2)(c) EPC, in combination with Art. 99(1) EPC, was substantive in nature, and called for reasoning which went to the merits of the opponent's case. A well-drafted opposition ought to contain reasoning that was full but concise. And in general, the less reasoning that a notice of opposition contained, the greater the risk that it would be rejected as inadmissible (similarly T 925/91, OJ 1995, 469; see also T 2/89, OJ 1991, 51; T 448/89, OJ 1992, 361; T 545/91, T 204/91).
In T 204/91 the board stated in particular that the term "indication" in R. 76(2)(c) EPC had to be construed as requiring more than a mere hint at a number of possible attacks upon the patent and at the likely support for each such possible attack, which indication or hint might then be augmented by the subsequent late filing, possibly even at the appeal stage, of further evidence, arguments or other matter - even of fresh grounds of objection. Rather, the scope and depth of "indication" needed to be such as to enable the patentee and the opposition division to see clearly just what attack was being mounted against the patent, and what evidential support was being adduced for that attack. In other words, the patentee and the opposition division had to be put in a position of understanding clearly the nature of the objection submitted as well as the evidence and arguments in its support. This required the elaboration of the relevant circumstances of the case to such an extent that the patentee and the opposition division were able to form a definitive opinion on at least one ground of opposition raised, without the need to make further investigations (see T 453/87 and T 279/88). The fact that a patentee must be able to understand, without undue burden, the case made against his patent in the notice of opposition does not, however, exclude the possibility that he may have to undertake a certain amount of interpretation, as was held in T 199/92.
In T 934/99 the board stated that R. 76(2)(c) EPC did not imply the requirement of a logical line of reasoning in the sense that the arguments brought forward in the notice of opposition had to be cogent or convincing. Rather, the criterion was whether the arguments presented were relevant and, where necessary as the result of a reasonable interpretative effort, specific enough to allow a person skilled in the art to form a reasoned opinion of whether the line of reasoning on which the opponent apparently relies was (logically) correct ("convincing") or not (i.e. wrong).
The issue of the sufficiency of the notice of opposition in this respect had to be distinguished from the issue of the strength of the opponent's case. R. 76(2)(c) EPC does not prescribe a complete "indication of the facts, evidence and arguments presented in support" so as to permit a conclusive examination on that basis alone. For the admissibility of an opposition, the statements made in the opponent's pleadings need not even be true nor the arguments conclusive in themselves. It is merely necessary that the opposition division and the patent proprietor are able to follow them. As a consequence, an (unfounded) ground of opposition may be clearly presented and argued. Conversely, an unconvincing submission may be rejected as not allowable even though, if properly drafted, it would have succeeded. The corroborative value of facts and evidence, however, is no longer an issue of admissibility but purely a question of the merits of the case (see e.g. T 621/91, T 3/95, T 152/95, T 1097/98, T 1069/96, T 1856/11; cf. T 534/98, T 353/06).
In T 426/08, the board examined the respondent's argument that, although evidence need not be produced but merely indicated by expiry of the opposition period, the admissibility of the opposition was not determined solely on the basis of the notice of opposition, but rather on the basis of the notice and the evidence indicated in it and produced either before or after the opposition deadline. Dealing with this argument, the board drew a distinction between evidence produced before and after the deadline and between indication and proof. For the purpose of examining admissibility, the board could not take account of evidence indicated during the opposition period but not produced until after the deadline as either an indication or proof of the facts. That would run counter to the wording of the EPC provisions applicable to the opposition procedure.
In T 1022/09 the opposition division had confused the criteria for admissibility of late-filed documents with those for documents filed with and referred to in an admissible notice of opposition. According to the board, the content and publication date of the documents filed by the opponent along with its notice of opposition did not come into play for the question of admissibility. Since their filing was substantiated, they were automatically admitted to the opposition proceedings. This fact could not be changed retrospectively by the subsequent withdrawal of representation by the professional representative.