According to Art. 99(1) EPC the notice of opposition must be filed within nine months of the publication of the mention of the grant of the European patent in the European Patent Bulletin. In the interests of legal certainty, the Convention lays down certain requirements for an opposition that must be met before the opposition period expires. These include the requirements that the opponent be sufficiently identified, the patent opposed be indicated, the notice of opposition be in writing, specific grounds be stated and the opposition fee be paid. Until the requirements of Art. 99(1) EPC regarding the filing of an opposition in due time have been met, a potential opponent is not a party to any proceedings before the EPO (see e.g. T 152/85, OJ 1987, 191).
The opposition period is triggered solely by the grant of a European patent and publication of the mention of its grant in the European Patent Bulletin, and not by publication of the patent specification or, for that matter, by publication of any subsequent correction of that specification. It also follows from the systematic relationship between Art. 98 and 99 EPC 1973 that the wording of the published specification has no legal effect whatsoever, even where it differs in substance from the grant decision. In particular, a mention published in the European Patent Bulletin that a correction of the patent specification has been issued does not trigger either a first or any "further" opposition period, even if the corrected specification confers a broader scope of protection than that conferred by the one originally published. Since the EPC does not provide for the re-establishment of the opponent's rights with respect to the deadline for filing a notice of opposition, it has to examined whether the failure to observe the time limit cannot be legally "cured" on the basis of the principle of legitimate expectations, even if the appellant has refrained from filing a notice of opposition because it had relied on the accuracy of the specification published by the EPO (cf. T 1644/10).
In T 438/87 the question addressed was whether a delay in publication of the European patent specification could in any way affect calculation of the time limit for filing an opposition. The board of appeal concluded that, in the case before it, the date of publication was the point from which the opposition period began, because the mention of the grant of the patent in suit had been brought to the public's attention in the normal way, irrespective of what problems may have affected publication of the patent specification and what consequences such untoward event might have for certain third parties owing to their geographical location.