Art. 99(1), last sentence, EPC states that a notice of opposition is not deemed to have been filed until the opposition fee has been paid. According to the board in T 152/85 (OJ 1987, 191), payment of an opposition fee is a factual requirement, which must be fulfilled within the nine-month opposition period if an opposition is to be admitted. Failure to pay an opposition fee within the required period where the filing of an opposition is intended is a factual mistake which cannot be rectified once the time limit for filing an opposition has passed. In the board's view, it could not be corrected under R. 88 EPC 1973 (R. 139 EPC). It was apparent from the wording of that rule that the clear policy of the EPO was to distinguish between a mistake in a document and other kinds of mistakes, such as failure to pay a fee in time, and only to allow correction of mistakes in a document. Art. 99(1) EPC 1973 provided a nine-month period during which notice of opposition could be filed and the opposition fee paid, payment of the fee being essential for the opposition. Until the requirements of Art. 99(1) EPC 1973 were met, a potential opponent was not party to any proceedings before the EPO.