Post-grant opposition takes the form of contentious proceedings between parties who normally represent opposing interests and who should be treated equally fairly. As soon as the notice of opposition has been filed, the procedure automatically becomes bilateral, no matter whether the opposition is valid, admissible or allowable. R. 79(1) EPC (former R. 57(1) EPC 1973) expressly requires the opposition division to communicate the opposition to the proprietor of the patent, and the Guidelines D-IV, 1.5 provide that communications and decisions about whether the opposition is deemed to have been filed and is admissible are also to be notified to the patent proprietor. Thus, the Convention does not provide for a decision by an opposition division "to end the ex parte proceedings" (T 263/00).
The opposition procedure is a purely administrative procedure which is initiated only on request, i.e. by the filing of opposition within the prescribed period. It thus differs fundamentally from the appeal procedure, which must be regarded as a judicial procedure and is, by its very nature, less investigative (see G 7/91 and G 8/91, OJ 1993, 356 and 346; G 9/91, OJ 1993, 408). In opposition proceedings, the EPO must investigate the facts of its own motion. However, this principle of ex officio examination is subject to a variety of restrictions designed to improve procedural transparency and efficiency, prevent delays by concentrating the proceedings and limit the risks run by the parties.
Under the EPC, there is no legal obligation for the patent proprietor to take an active part in opposition proceedings. However, the patent proprietor is not free to present or complete his case at any time that he wishes during the opposition or opposition appeal proceedings, depending, for example, on his procedural strategy or his financial situation. In view of the judicial nature and purpose of inter partes appeal proceedings and in the interests of an efficient and fair procedure, it is necessary that all parties to opposition proceedings complete their submissions during the first-instance proceedings in so far as this is possible. If a patent proprietor chooses not to respond in substance at all to the opposition, for example by filing arguments or amended claims, or chooses not to complete his submissions at the stage of the first-instance proceedings, but rather presents or completes his case only in the notice of appeal or the statement setting out the grounds of appeal, then he will need to face the prospect of being held to account for such conduct by the board when, for example, exercising its discretion under Art. 12(4) RPBA. This applies in particular if all the reasons for revocation of the opposed patent were known to the patent proprietor before it received the impugned decision (T 936/09).
In the parallel cases T 1553/06 and T 2/09 involving the same parties but different patents, the same board dealt with the admissibility of the respective oppositions filed within the framework of a test case. In particular, the board examined whether the parties and their representatives had worked together on a test case in order to obtain answers from the EPO to specific legal questions regarding prior art. To establish the admissibility of its oppositions, the opponent (appellant) relied basically on the argument that the contentious character of the opposition proceedings was not a general principle and, even if so, the parties in the case at issue satisfied the criteria for "contentious proceedings". The existing co-operation between the parties' representatives on the test case that arose out of a discussion in a study committee did not render the proceedings non-contentious.
Following the principles set out in G 9/93 and G 3/97, the board emphasised the contentious nature of opposition proceedings as a necessary condition for the admissibility of the opposition and examined whether an abuse of procedure, i.e., because the opponent acted on behalf of the patent proprietor ("straw man"), rendered the opposition inadmissible in the case before it. The board could not find a circumvention of the law by abuse of process as it saw no reason to cast doubt on the parties' submissions that the opponent was not bound by any instructions from the patentee or the study committee. As to the further question whether the opposition proceedings were deprived of their contentious character due to the very fact that the parties defended their positions within the framework of a test case, the board did not see any reason to question that the proceedings were contentious in nature. An opposition filed within the framework of a test case is not inadmissible for that sole reason, provided that the prosecution of the proceedings thereby instituted is contentious because the parties defend mainly opposing positions (see point 1.3 of the Reasons in both cases). The respective oppositions were therefore deemed admissible.