Revocation of a patent concerns a mechanism which provides for an inter partes review of the validity of a. At the European Patent Office a patent can be revoked by the Opposition Division or Boards of Appeal. A notice of opposition has to be filed within nine months from the publication of the mention of the grant of a European patent.  In 2000 Article 105a was introduced in the European Patent Convention, providing centralised limitation and revocation procedures at the level of the EPO, which allows the patent proprietor either to have the claims of his granted patent limited or to have the whole patent revoked for all its designated states. The effect of this central revocation is that the patent in question is deemed never to have existed. It is revoked ex tunc.