If the claimed priority dates cannot be verified at this stage, uncertainty will exist as regards their validity and the search for conflicting applications is extended so as to cover all published applications with an earliest claimed priority date up to the filing date (not the claimed priority date(s)) of the application under consideration (see B‑IV, 2.3, and B‑XI, 4).
Source: http://www.epo.org/law-practice/legal-texts/html/guidelines/e/b_vi_3.htm
Date retrieved: 17 May 2021