It is the responsibility of the examining division to check whether and to what extent the priority claim is justified. However, where intervening state of the art (see B‑VI, 5.2) or potential state of the art according to Art. 54(3) is revealed in the search, the search division checks, if possible, the validity of the priority claim (see B‑XI, 4, F‑VI, 1.2, to F‑VI, 1.5, and F‑VI, 2). Furthermore, if a document showing that a priority claim might not be justified (e.g. an earlier application or patent from the same applicant indicating that the application from which priority is claimed may not be the first application for the invention concerned) is found during the search, it will be cited in the search report (see B‑X, 9.2.8). However, no special search effort is normally made for this purpose, except when there is a special reason to do so, e.g. when the priority application is a "continuation-in-part" of an earlier application from which no priority is claimed (see B‑IV, 2.3, and F‑VI, 2.4.4). Sometimes the fact that the country of residence of the applicant is different from the country of the priority application may also be an indication that it is not a first filing, justifying a certain extension of the search.

When the search is extended for this purpose, it is directed to:

Example 1 (assuming that the applicant is the same for all applications):

application:

GB1 filed

GB2 filed

EP1 filed
(claiming priority of GB2)

GB1 published

During the search for EP1, the search division retrieved published application GB1. GB1 may prejudice the priority claim of EP1, since it was filed earlier than GB2. Published GB1 is, therefore, cited in the search report as an "L" document according to B‑X, 9.2.8(a); or

Example 2 (assuming that the applicant is the same for all applications):

application:

GB1 filed

GB2 filed

US1 filed
(claiming priority of GB1)

EP1 filed
(claiming priority of GB2)

US1 published

The publication US1 was found during the search for EP1. GB1 may prejudice the priority of EP1, since it was filed earlier than GB2. US1, which claims GB1 as priority, is, therefore, cited in the search report as an "L" document according to B‑X, 9.2.8(a).