An international application may claim the priority of an earlier application that was filed in any state party to the Paris Convention for the Protection of Industrial Property or for any member of the World Trade Organization (see points 2.13.001 ff). The file number of the earlier application, if known at the time of filing the international application, is to be provided in Box VI of the PCT request form. In the proceedings before the EPO as designated/elected Office, the priority application is referred to as the "previous application", which corresponds to the terminology used in the EPC.

The applicant is required to provide the file number and to file a certified copy of the previous application with the receiving Office or the IB during the international phase, i.e. within 16 months of the (earliest) priority date (see point 2.15.001), so that both will be available to the EPO on entry into the European phase (see points 2.15.001 ff and point 5.13.008). The IB will normally forward the certified copy of the previous application to the EPO prior to expiry of the 31-month time limit. It will also upload the priority document to its PATENTSCOPE database, where it can be accessed by the EPO. If the EPO receives the priority document from the IB or has access to it via PATENTSCOPE, the applicant does not need to furnish it to the EPO again.

If the international application was filed outside the priority period, a request for restoration of the right of priority may have to be (re)filed with the EPO as designated/elected Office (see points 5.13.017 ff).