An actual party to the proceedings, such as an applicant, may not however file such observations, see Guidelines for Examination in the EPO, section e-vi, 3 : "Observations by third parties".
Decision T 156/84 of April 9, 1987, reasons 3.5, first paragraph: "(...) the EPO also has a duty vis-à-vis the public not to grant or maintain patents which it is convinced are not legally valid"; Legal Research Service for the Boards of Appeal, European Patent Office, Case Law of the Boards of Appeal of the EPO (9th edition, July 2019), iii.n.1 , "Observations by third parties", "Introduction", last paragraph: "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)."
