Under Art. 115 EPC, any third party may present, following publication of the European patent application, observations concerning the patentability of the invention to which the application or patent relates. There are no fees or deadlines for presenting such observations. The third party is not a party to the proceedings. R. 114(1) EPC requires that any such observations be filed in an official language of the EPO, while R. 114(2) EPC states that they will be communicated to the applicant or patent proprietor. Thirdparty observations are an integral part of the files and, as such, open to inspection under Art. 128 EPC. More information can be found in the Guidelines (see e.g. A-VII, 4.2; A-XI, 2.1; D-X, 4.5 and E-V, 3 - June 2012 edition), the President's decision of 10 May 2011 and the Notice from the EPO dated 10 May 2011 concerning the filing of thirdparty observations (OJ 2011, 418).
Art. 115 EPC helps to ensure that no legally invalid patents are granted or maintained. The public must be able to rely as far as possible on the legal validity of a patent granted by the EPO and the provision is designed to protect this public interest (T 156/84, OJ 1988, 372; T 60/91, OJ 1993, 551; T 1495/09).