The right of priority is generally regarded as one of the cornerstones of the Paris Convention. Its basic purpose is to safeguard, for a limited period, the interests of a patent applicant in his endeavour to obtain international protection for his invention, thereby alleviating the negative consequences of the principle of territoriality (T 15/01, OJ 2006, 153).
Art. 87 to 89 EPC provide a complete, self-contained code of rules on claiming priority for the purpose of filing a European patent application (see J 15/80, OJ 1981, 213, most recently confirmed in J 9/07). However, since the EPC - according to its preamble - constitutes a special agreement within the meaning of Art. 19 of the Paris Convention, it is clearly intended not to contravene the basic principles concerning priority laid down in the latter (see T 301/87, OJ 1990, 335; G 3/93, OJ 1995, 18 and G 2/98, OJ 2001, 413). Furthermore, in view of Art. 87(1) and (2) EPC, the priority system adopted by the European legislator also recognises "internal priorities" (T 15/01, OJ 2006, 153).
The EPO does not normally check the validity of a priority right during examination. A check must be made, however, if relevant prior art has been made available to the public within the meaning of Art. 54(2) EPC on or after the priority date claimed and before the date of filing or if the content of the European patent application is totally or partially identical with the content of another European application within the meaning of Art. 54(3) EPC, such other application claiming a priority date within the above-mentioned period (Guidelines F-VI, 2.1 - June 2012 version). In opposition proceedings this applies where prior art is invoked in connection with a ground for opposition under Art. 100(a) EPC in relation to which the priority date is of decisive importance (Guidelines D-III, 5 – June 2012 version). Failure by the division to check the validity of the priority right in the circumstances described has been held to be a substantial procedural violation (see T 16/89, T 737/95, T 2170/08).