Lack of unity may be directly evident a priori, i.e. before examination of the merits of the claims in comparison with the state of the art revealed by the search (W 1/96 and W 6/90, OJ 1991, 438). If the objection of lack of unity is raised a priori, the technical problem must be defined solely on the basis of the description and not of the prior art (see W 50/91, W 52/91, W 22/92, W 52/92 and T 188/04).

On the question whether the International Searching Authority (ISA) had the power to assess unity of invention in an international application a posteriori, i.e. after taking into account the prior art brought to light by the search, the board in W 3/88 (OJ 1990, 126) answered in the negative, interpreting the expression "a single general inventive concept" in R. 13.1 PCT as being simply the general concept of what the applicant subjectively claimed to be his invention. By contrast, in W 44/88 (OJ 1990, 140) and W 35/88, the answer was affirmative. In the latter cases the boards referred to the PCT Search Guidelines, which permitted assessment of unity a posteriori. These conflicting interpretations gave rise to referrals under Art. 112 EPC 1973 by a board of appeal (W 12/89 date: 1989-06-29, OJ 1990, 152) and by the President of the EPO.

The referred questions, dealt with in consolidated proceedings, were answered in G 1/89 (OJ 1991, 155) and G 2/89 (OJ 1991, 166) respectively. According to the Enlarged Board, the EPO in its capacity as an ISA might, pursuant to Art. 17(3)(a) PCT, request a further search fee where the international application was considered to lack unity a posteriori. The Enlarged Board first noted that the problems involved were mainly due to the fact that, under the PCT, as well as under the EPC 1973, search and (substantive) examination were carried out in sequentially different steps and by different examiners. This procedural separation of search and examination led inevitably to a certain overlapping because of the functional relationship between search and examination. Thus, although the objective of the search was in principle limited to discovering and reporting on relevant prior art for the purpose of assessing novelty and inventive step, which was ultimately the task of the examining authority (i.e. the IPEA and/or the designated Office under the PCT and the examining division under the EPC 1973 respectively), it was obvious that in many instances the search examiner needed to form a provisional opinion on these issues in order to carry out an effective search. Otherwise, he would simply be at a loss how to judge the relevance of the documents in the prior art and how to arrange the search report accordingly. (See also in this chapter II.B.4.2.)

The Enlarged Board further noted that the requirement of unity of invention under the PCT applied equally to the procedure before the ISA and to the procedure before the IPEA according to Art. 17(3)(a) and 34(3)(a) PCT respectively, which was in conformity with the procedural separation of search and examination as referred to above and reflected the principle that the normal fees for search and examination were related to one invention (or a single general inventive concept) only. This led to the conclusion that the requirement of unity of invention under the PCT had in principle to be judged by the same objective criteria by both the ISA and the IPEA.

The Enlarged Board observed that the PCT Search Guidelines contained a direct reference to the consideration of unity by the ISA on an a posteriori basis, i.e. after an assessment of the claims with regard to novelty and inventive step in relation to the prior art. A comparison with the corresponding EPO Guidelines showed that under the EPC 1973 it was also clearly foreseen that consideration of unity by the search divisions might be carried out on an a posteriori basis (see also R. 46 EPC 1973; now R. 64 EPC). In the view of the Enlarged Board, this was a consequence of the special structure of the PCT and the EPC 1973. Therefore, the Guidelines had on this point to be considered as consistent with the PCT and the EPC 1973. As to the PCT Search Guidelines, it was noted that such guidelines were based on Art. 56 PCT dealing with the task of the Committee for Technical Cooperation (see in particular Art. 56(3)(ii) PCT and the reference to the need for uniformity of, inter alia, working methods; see now Chapter GL/ISPE 10 of the PCT International Search and Preliminary Examination Guidelines, as in force from 1.1.2019).

According to the board in W 21/89, since, under the terms of R. 33.1(a) PCT, the prior art consisted of everything that had been made available to the public before the date of filing of the international application, the ISA could use such documents as grounds for a decision of lack of unity a posteriori even if it was not possible to cite these documents against a corresponding national patent application in one of the contracting states designated.

The principle whereby the EPO may also find a lack of unity "a posteriori" when carrying out an international search (G 1/89, OJ 1991, 155; G 2/89, OJ 1991, 166) applies equally to a European search because in both cases the searches and search reports are virtually identical (see T 87/88, OJ 1993, 430).

In W 2/10 the board pointed out that in W 4/85 (OJ 1987, 63) and many subsequent decisions, the boards had decided that the requirement to give reasons in an invitation under Art. 17(3)(a) PCT was so fundamental that an unsubstantiated invitation could be regarded as legally ineffective. This was so in the case in question and therefore the additional search fees had to be refunded.