Discretion of the opposition division in selecting the closest prior art

It is not possible to determine in general terms how many starting points an opposition division must take into account when examining inventive step as the fictitious closest prior art in the "problem-solution approach". The answer to this question depends on circumstances of the individual case, in particular on which distinguishing features the various starting points exhibit. However, it is clear that in practice only a limited number of lines of attack can be examined. Therefore, an opponent is encouraged to select from the large number of possible starting points those which, in their view, most clearly challenge the patentability of the claimed subject-matter. If the number of attacks exceeds a reasonable level, an opposition division must have some discretion to not consider any lines of attack which are not promising or which are clearly less promising, such that the proceedings do not get out of hand. However, the opposition division must give the opponent the opportunity during oral proceedings to comment on a document as a starting point for an attack on inventive step if this document was discussed in the preliminary opinion of the opposition division issued with the summons as a relevant possible starting point for the "problem-solution approach". Denying the opponent this opportunity constitutes a violation of the right to be heard and thus a fundamental procedural defect (EPO Board of Appeal 11/4/2022, T 2610/19).