• the extent of opposition (which claims are opposed),
• the grounds of opposition, and must
• indicate the evidence (e.g. prior art), facts and arguments upon  hich the opposition is based.

To be admissible, the proprietor and Opposition Division must be able to understand, from the opposition papers, the case to be answered.

Grounds of opposition
There are only three possible grounds of opposition:

• the claimed invention is not patentable (either because the subject matter is excluded from patentability or because it lacks novelty or inventive step);
• the specification of the patent does not disclose the invention sufficiently clearly and completely for it to be carried out by a person skilled in the art (“insufficiency”), and
• the subject matter of the European patent extends beyond the content of the European application as originally filed (“added subject matter”).

Initial procedure - observations of patentee
Oppositions are handled by what is usually a three-member Opposition Division. The Notice of Opposition is initially communicated to the proprietor and after it has been examined as to its admissibility the EPO set a four month period within which the proprietor is invited to file observations on the opposition. This four month term is extensible.

The deadline is not fatal as not filing any observations does not automatically result in revocation of the patent. The EPO will, in any event, examine the opposition of their own motion and come to a decision on the basis of whatever the opponent and the proprietor themselves submit from the filing of the opposition onwards.

The proprietor’s observations are then communicated to the opponent and thereafter there is no real set procedure. Indeed, should they so wish, the Opposition Division may, shortly after this communication, give an immediate decision.

As a precaution, therefore, an opponent or proprietor should, with his first submission, request a hearing (“oral proceedings”) in the event that the Opposition Division should form an intention to issue a decision adverse to him (see also below). However, generally the EPO will invite the further submissions of the parties to the extent and the number of times they feel it necessary to enable them to come to a preliminary view on the case.

Each party should receive the same number of opportunities to file comments and further arguments to supplement what has gone before.

At some point, a preliminary and non-binding opinion will often be issued by the EPO, allowing the parties to see how the merits of their respective cases have been received by the EPO.

Requests
Frequently, the proprietor will put in, either at the observation stage or perhaps even in the later stages of the opposition procedure, certain ‘requests’, called ‘main’, ‘first subsidiary’, (or ‘first auxiliary’), ‘second auxiliary’, et seq., requests. These represent a series of fall-back positions by way of amended claims which the proprietor is entitled to have examined sequentially by the EPO.

However, the sequence of the requests must be carefully chosen as subsequent requests are only examined after the preceding request has been found not to be allowable. If one of these requests is found to be allowable, the patent would then be maintained on the basis of that allowed request.

Late filed evidence/requests
It is advisable that whatever is to be introduced into the opposition proceedings, e.g. prior art, is submitted at the beginning with the Notice of Opposition. Likewise, requests of the proprietor are best filed at the observation stage. Strictly speaking, admittance or not into the proceedings of any evidence filed after that is at the discretion of the EPO. If it is of “crucial relevance”, then it will usually be admitted; it is a fundamental EPO principle that patents of doubtful validity should not be allowed to remain against the public interest.

However, the less relevant and the later such late-filed additional evidence is submitted, the less likely that it will be admitted. Thus, when contemplating the filing of an opposition it is important to commence preparation of whatever evidence is to be filed, including, for example, searching, generating experimental data, etc., as early as possible prior to expiry of the opposition period.