The principle of the protection of legitimate expectations is a general principle well established in European Union law and generally recognised in the EPC contracting states and board of appeal case law (G 2/97, OJ 1999, 123; see also R 4/09). Its application to procedures before the EPO implies that measures taken by the EPO should not violate the reasonable expectations of parties to such proceedings (G 5/88; G 7/88; G 8/88, OJ 1991, 137). The term "good faith" is also used to describe this concept (J 10/84, OJ 1985, 71; J 38/97; J 19/13; J 19/16).

The protection of the legitimate expectations of users of the European patent system has two main principles. It requires that the user must not suffer a disadvantage as a result of having relied on erroneous information or a misleading communication received from the EPO (see in this chapter III.A.2.). It also requires the EPO to warn the applicant of any loss of right if such a warning can be expected in good faith. This presupposes that the deficiency can be readily identified by the EPO (see in this chapter III.A.3.).

Users of the European patent system, who are parties in proceedings before the EPO, must also act in good faith (G 2/97, R 4/09, T 861/12). An alleged violation of the principle of the protection of legitimate expectations is in itself not a ground for a petition for review under Art. 112a EPC (R 13/11, R 1/16).