The patent, for technology known as "an access time register," was granted to US-based Rambus on the basis of an application filed in 1991. It covered Germany, France, the UK and Italy.
Opposition to the patent was filed in 2000; but in September 2002 the EPO affirmed the novelty and inventiveness of the patent, rejecting numerous "prior art" arguments raised by the rival major semiconductor manufacturers.
However, the EPO also said that the patent filing would only be valid with further amendment by Rambus to conform more closely to the language as originally filed. This last aspect of the ruling, which reversed a prior EPO decision, was appealed by Rambus.
On Thursday, after a three-day hearing, the Technical Board of Appeal of the EPO announced that the patent had been revoked. The written ruling has not yet been published, and is not expected to be available for another four to ten weeks, in accordance with EPO procedure.
Rambus was quick to point out that the patent relates only to one aspect of its patented technologies and that the ruling would not affect other patents and patent applications held by the company.
Rambus is already suing chip makers both in the US and in Europe for refusing to pay royalties on its technology.
Micron Technologies, one target of the patent litigation, disagrees on the significance of the EPO revocation. A spokesman told Reuters on Friday: "This decision should result in the termination of a number of cases filed by Rambus against Micron in Europe".
He added, "There are additional oppositions against related Rambus patents currently pending in the EPO, and we expect the EPO to apply the same reasoning with respect to those patents".