In principle, only the first application filed in a state party to the Paris Convention or a member of the WTO can form the basis of a priority right. In the EPC, this is made clear in Art. 87(1) and (4) EPC.
Therefore, if apart from the application whose priority is being claimed in the subsequent European application, an earlier previous application was also filed, in order to check the validity of the priority claim it must be established whether the invention claimed in the subsequent application was already disclosed in the earlier previous application (see point 1 above). Two more recent decisions concerning the question whether the priority application or a still earlier application was the first application also focused on the identity of the applicants (see point 2 above).
In T 477/06 the board held first that since Euro-PCT application D8 had been deemed withdrawn due to non-payment of the designation fees (R. 23a EPC 1973), it was not prior art under Art. 54(3), (4) EPC 1973 for the application in suit. However, it had been filed by the same applicant and earlier than the priority application. In view of the fact that claimed subject-matter was anticipated by this earlier application, the priority application was not the first application within the terms of Art. 87(1) and (4) EPC 1973 (cf. Art. 8(2)(b) PCT), so that the priority claim was not valid. Hence, the application in suit enjoyed priority only from the date of filing and D8 was prior art under Art. 54(2) EPC 1973. See also T 1222/11.
For the case of a priority held invalid because it was not shown, for the purpose of Art. 87(4) EPC 1973, that at the date of filing of the application from which priority was claimed, a previous first application filed by the applicant in respect of the same invention had been withdrawn, see T 1056/01 (reported in Chapter III.G.4.3.4, Law of evidence).