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 thus 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. Nor could the failure to pay the opposition fee in time be rectified on the basis of R. 139 EPC (former R. 88 EPC 1973). It was apparent from the wording of R. 139 EPC (former R. 88 EPC 1973) 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.