A granted European patent enjoys the legal presumption that the invention is disclosed in a manner sufficiently clear and complete to be carried out by a person skilled in the art using the information contained in the specification and, if applicable, the common general knowledge. In opposition proceedings, this legal presumption can be rebutted only if the opponent’s submission raises serious doubts about the presumption and these doubts need to be substantiated by verifiable facts (cf. decision T 19/90 of Oktober 3, 1990, No. 3.3 of the reasons or T 0967/09 of November 4, 2014, No. 6 of the reasons). This requires the opponent to present facts and arguments and to substantiate why these facts and arguments justify the lack of sufficiency. If the alleged facts are disputed, the opponent must prove them. He bears the burden of proof which means that remaining doubts are for the benefit of the proprietor and will lead to a rejection of the ground of opposition.

If the arguments presented by the opponent are conclusive and his alleged facts are sufficiently substantiated by evidence (or undisputed), the legal presumption is rebutted, and the burden of proof is reversed. It is now up to the proprietor to prove that the invention can be carried out (for example by providing experimental data / test results) contrary to the opponent’s assertions. Any remaining doubts are now at the expense of the proprietor and will help the attack on sufficiency to success.

In a recent case before the Board of Appeal 3.3.05 (decision T 1076/21 of May 26, 2023), an opponent had successfully argued against the sufficiency of the disclosure and the patent had been revoked in the first instance based on this ground. In subsequent appeal proceedings, a major question was whether the first instance decision had led to an automatic rebuttal of the legal presumption of sufficiency and whether the burden of proof with respect to the sufficiency had shifted to the patent proprietor. However, the Board of Appeal denied this and clarified that formal reasons, such as the existence of a first instance decision, cannot cause a reversal of the burden of proof, but that the reversal depends solely on the existence of serious doubts, supported by verifiable facts.

The patent proprietor successfully argued that the opposition division’s reasoning leading to the revocation of the patent was wrong and that the opponent did not adequately substantiate the alleged lack of sufficiency by verifiable facts. While the parties made contradictory assertions which were not sufficiently evidenced by any one of them, the Board emphasized that the burden of proof still lay with the opponent. Accordingly, the doubts about the facts were to the detriment of the opponent and, consequently, the first instance decision was set aside.

Moreover, an expert opinion submitted by the opponent in response to the preliminary opinion of the Board of Appeal was considered late filed and was not admitted into the proceedings.

The decision shows once again that the assertion of lack of sufficiency requires considerable efforts, which opponents should already make in preparation of the first instance proceedings to be able to substantiate the lack of sufficiency, for example by submitting own experimental data or test results.