1. a) A European patent application only validly claims the priority of an earlier application in respect of subject matter which is directly and unambiguously derivable for a person skilled in the art from the priority application.   If, during proceedings before the EPO, any document is cited that has an effective or publication date between the priority date and the EP filing date, it is necessary to assess whether the EP application validly claims priority. If the priority application is not in one of the EPO's official languages, Rule 53(3) EPC confers on the EPO the right to request that the applicant/proprietor file a translation of the priority application into one of the EPO's official languages.   b) Rule 53(3)EPC in force until March 30th 2013 did not specify a specific legal sanction where the applicant/proprietor did not comply with the EPO's request for a translation. However, the EPO Guidelines for Examination indicated that non-compliance would result in the intermediate document being considered to belong to the prior art (Guidelines A III-14, 6.8).   2. a) Rule 53(3)EPC has been amended, with effect from April 1st 2013, in order to extend the legal effect of non-compliance with the EPO's request. The amended provision applies to invitations under Rule 53(3)EPC issued on or after April 1st, 2013.   b) The following phrase has been added to Rule 53(3) as amended: "If a requested translation of a previous application is not filed in due time, the right of priority for the European patent application or for the European patent with respect to that application shall be lost. The applicant for or proprietor of the European patent shall be informed accordingly." 3. Therefore, in order to avoid loss of priority entitlement, it is now necessary to comply promptly with the EPO's invitation, either by filing the requested translation or where applicable indicating that the EP application is a complete translation of the priority application.   4. More detailed information is available at: http://www.epo.org/law-practice/legal-texts/official-journal/information-epo/archive/20130227a.html.   If you have any questions with regard to Rule 53(3) EPC, please contact Oliver Tischner.   This IP ALERT is for information purposes only and does not constitute legal advice.   