Parties to proceedings before the EPO – and their representatives – are expected to know the relevant provisions of the EPC, even when such provisions were intricate (J 27/92, OJ 1995, 288; T 578/14; J 10/17), as well as all notices published by the EPO which are relevant to patent practice (T 267/08). In general, the parties to EPO proceedings are presumed to know the law relating to the EPC, including the relevant decisions of the boards of appeal (R 17/09).

The board decided in J 17/98 (OJ 2000, 399) that the principle of protection of legitimate expectations according to which communications of the EPO, including official forms, must be clear and unambiguous, did not extend so far as to require comprehensive legal advice to be contained in such forms. While forms must be clear and unambiguous, they need not contain detailed explanations of the law. This was especially true for legal issues which directly follow from the provisions of the EPC (see also T 778/00, OJ 2001, 554).

The board decided in J 5/02 that it was not a violation of the principle of the protection of legitimate expectations if the EPO provided a professional representative with incorrect information on the basis of which he concluded that the relevant legal provisions – in this case Art. 122 EPC 1973 – were no longer applicable. If he did not realise the information was incorrect, he was guilty of fundamentally inexcusable ignorance of the law; if he did realise it was incorrect, he was not misled.

In T 267/08 the board held that professional representatives must be expected to be acquainted with all notices published by the EPO which are relevant to patent practice. The new representative should have realised that the Decision of the President of the European Patent Office dated 12 July 2007 on the filing of authorisations (OJ SE 3/2007, L.1) explicitly required an original version of the authorisation to be filed and that the decision of the President of the European Patent Office dated 12 July 2007 concerning the filing of patent applications and other documents by facsimile (OJ SE 3/2007, A.3) forbade the filing of authorisations by fax. The fact that the new representative had previously been notified that the change of representative had been registered should not have misled him into believing that a faxed version of an authorisation would suffice. Only a "fundamentally inexcusable ignorance of the law" (referring to J 5/02) could lead him to this conclusion.

In T 590/18 of 4 July 2018 the board held that a debit order filed on paper (EPO Form 1010) after 1 December 2017 could at most be accepted as a valid payment of the appeal fee if the appellant could successfully claim to have seen still current information clearly indicating the option of paying this way on the EPO website after the entry into force of the change to fee payment methods, to have been entitled to entertain a legitimate expectation as to that information's accuracy, and indeed to have acted in reliance on it. Finding a PDF version of a brochure published before the change's entry into force was not sufficient to establish such a legitimate expectation, especially when the appellant had anyway known about the change.

The Enlarged Board held in R 4/09, in the context of an alleged breach of the principle of legitimate expectation, that a party to the proceedings, at least where it was represented by an authorised representative, was deemed to know the case law, and cannot plead lack of such knowledge as an excuse, "ignorantia legis non excusat" (see also T 736/14). According to J 19/10, the professional representative could be expected to be familiar with procedural matters in general, and the boards' detailed and consistent jurisprudence in particular.

In T 1086/09 the board held that the representative's expectation that a refund of 50% of the appeal fee would be available was not legitimate as it was the consequence of an error in law, i.e. an incorrect reading of the new R. 103(2) EPC, which did not apply to the appeal in question.