In T 1201/14 the board observed with regard to the formal requirements for the transfer of the right of priority that, as the EPC contained neither guidance nor conflict-of-law provisions for that purpose, national law was commonly relied upon by the departments of the EPO entrusted with the procedure. The board also acknowledged that there was no established jurisprudence of the boards as to the national law generally applicable to this question.

In T 1008/96 the European patent application resulting in the patent in suit and the two Italian utility model applications from which priority was claimed had been filed by different persons. The board held that the succession in title had to be proven to its satisfaction. It was a general principle of procedural law that any party claiming a right had to be able to show that it was entitled to that right (see J 19/87). This question had to be answered in accordance with national law. The board concluded that the respondent had failed to prove that it was the successor in title in respect of the two Italian utility model applications. Consequently, no priority rights existed for the patent in suit.

In T 62/05 the board pointed out that the EPC did not contain any regulations on the formal requirements that an assignment of priority rights for the filing of a European patent application should fulfil in order to be considered valid for Art. 87(1) EPC 1973. However, having regard to the crucial effect that a valid priority date has on patentability (cf. Art. 89 EPC 1973), and to the fact that, in the case at issue, the validity of the priority claim depended on the validity of the transfer of the ownership of the right of claiming priority from the Japanese first filing for the European patent application, such transfer of priority rights, in the board's view, had to be proven in a formal way (by analogy with T 1056/01 – also summarised in chapter III.G.4.3.5). It hence appeared reasonable to the board to apply an equally high standard of proof as that required for the assignment of a European patent application by Art. 72 EPC 1973, which, however, the board concluded was not met here. Furthermore, the appellant could not convince the board that an assignment had taken place implicitly and tacitly before the end of the twelve-month period or that it would have been illogical for the applicant of the priority application to assign the application to the applicant for the patent in suit without assigning the corresponding priority rights.

In T 160/13 the board confirmed that the opposition division had rightly examined the transfer of the priority right on the basis of German law, which called for a bilateral declaration of transfer by both applicants (§§ 398 and 413 in conjunction with §§ 145 ff. of the German Civil Code (BGB)). Such declarations were not subject to any particular formal requirements under German law, and no other requirements of a formal nature could be inferred from the decisions cited by the appellant, T 1056/01 and T 62/05. In the board's view the correspondence on file provided ample proof of the transfer.

According to the board in T 577/11, where the applicant of the priority application and the applicant of the subsequent application contractually agree that (only) economic ownership ("economische eigendom" under Dutch law) of the priority application and the right to claim its priority is to be transferred to the subsequent applicant, this is not sufficient to consider the latter a successor in title within the meaning of Art. 87(1) EPC 1973 (see decision catchword). The board recognised the retroactive effect of the transfer of “economic ownership” but not of the legal title. It concluded that at the time of filing of the subsequent application this arrangement only amounted to a limited transfer, insufficient for the purposes of Art. 87(1) EPC 1973. Furthermore, this case was to be distinguished from the situation of equitable assignment under English law, which was the subject of J 19/87.

Considering the requirement that the right of priority must have been transferred before the filing date of the later European application, the board in T 1201/14 took the view that, even if a retroactive transfer such as the nunc pro tunc assignment under US law invoked by the appellant was allowable under US law, it would not be acceptable under Art. 87(1) EPC 1973. The board also held that an implied transfer of a particular right could be accepted when it was sufficiently clear that the parties had formed an agreement and what they had agreed. The burden of proving a valid transfer of the right of priority lay with the proprietor since it was the one claiming that right. As to the standard of proof to be applied to an implied transfer by virtue of a general policy under German law, the board held that the circumstances of the case in hand required proof "beyond reasonable doubt", as all the relevant evidence lay within the knowledge and power of only one party to the inter partes proceedings.

In T 493/06 it was held that the respondents had provided sufficient proof of the assignment of priority rights. In the board's view even a copy of an assignment agreement could be sufficient provided that evidence was supplied that the content of the copy was identical to that of the original document. Such evidence might, according to board of appeal case law, take the form of an affidavit – even though affidavits were not expressly mentioned in Art. 117(1) EPC 1973 – and the principle of unfettered consideration of the evidence applied to them (see e.g. T 970/93, T 804/94, T 558/95 and T 43/00; also T 535/08).

According to the board in T 1103/15, if a party made statements about conclusions to be drawn on the basis of the applicable national law, it had to file suitable evidence, for example by filing as documents adequate copies of such laws and/or as appropriate by filing as expert evidence the opinions of a suitably qualified lawyer in the relevant jurisdiction (T 74/00).

The board in T 2466/13 observed that there was no clear line in the boards' case law on the standard of proof to be met for a transfer of a priority right to be considered valid (compare T 205/14 and T 517/14, applying the balance of probabilities, with T 1201/14, in which a stricter standard was applied).

See also T 404/13, T 725/14, as well as the decisions in chapter II.D.4.2.