Assumed infringers of a patent (see D‑I, 5) may file notice of intervention in the opposition proceedings within three months of the date on which infringement proceedings were instituted against them or on which they instituted proceedings for a court ruling that they are not infringing the patent. Notice of intervention must be filed in a written reasoned statement. It is not deemed to have been filed until the opposition fee has been paid in the amount prescribed in the Rules relating to Fees under the EPC.[Art. 105; Rule 89; ]

Intervention is permissible as long as opposition or appeal proceedings are pending. A third party can become a party to the proceedings during the period for filing an appeal only if a party to the proceedings in which the decision was given files an appeal pursuant to Art. 107; otherwise the decision of the opposition division will become final on expiry of the appeal period (see G 4/91 and G 1/94).

A properly filed and admissible intervention is treated as an opposition, which may be based on any ground for opposition under Art. 100 (see G 1/94). This means that, when intervening at any stage of first-instance proceedings, the intervener enjoys essentially the same rights as any other party to the proceedings. If the intervener introduces new facts and evidence which appear to be crucial, the proceedings may need to be prolonged to enable them to be adequately considered. In all other cases the opposition division must ensure that the intervention does not delay the proceedings.

If the notice of intervention is filed at a late stage of the proceedings, for example when oral proceedings have already been scheduled, the opposition division may dispense with issuing communications under Rule 79(1) to Rule 79(3). The introduction of a new ground for opposition at such a late stage may lead to a postponement of the date set for oral proceedings.[Rule 79(4); ]

For accelerated processing of oppositions and accelerated processing before the boards of appeal on request, see E‑VIII, 5, and E‑VIII 6.

The notice of intervention, filed in a written reasoned statement, must contain:[Rule 89(2); ]

(i)a statement of the grounds for intervention and corresponding evidence. The proceedings providing the grounds for intervention must be directed towards establishing an infringement (or its absence) as a final legal result. Proceedings directed at the preservation of evidence to enable a party to initiate separate infringement proceedings are not sufficient in this regard (see T 439/17).[Art. 105(1); ]

(ii)the name, address and nationality of the assumed infringer and the state in which the assumed infringer's residence or principal place of business is located. Names of natural persons must be indicated by the person's family name and given name(s), the family name being indicated before the given name(s). Names of legal entities, as well as companies considered to be legal entities by reason of the legislation to which they are subject, must be indicated by their official designations. Addresses must be indicated in such a way as to satisfy the customary requirements for prompt postal delivery at the indicated address. They must comprise all the relevant administrative units, including the house number, if any. Assumed infringers (whether natural or legal persons) whose residence or principal place of business is in an EPC contracting state and who act without a professional representative can use an address for correspondence other than their residence. The address for correspondence must be the assumed infringer's own address. Post cannot be sent to a different (natural or legal) person, since that requires a valid form of representation under Art. 133 and Art. 134. It is recommended that the telephone and fax number be indicated (see D‑IV, 1.2.2.2(i) and D-IV, 1.4.2);[Rule 76(2)(a); Rule 41(2)(c); ]

(iii)the number of the European patent at issue in the opposition proceedings in which intervention is made, the name of the patent proprietor and the title of the invention (see D‑IV, 1.2.2.2(ii) and D-IV, 1.4.2);[Rule 76(2)(b); ]

(iv)a statement of the extent to which the European patent at issue is opposed by way of intervention and of the grounds on which the opposition by way of intervention is based, as well as an indication of the facts and evidence presented in support of these grounds, together with a statement of reasons, i.e. arguments (see D‑IV, 1.2.2.1(iii) to D-IV, 1.2.2.1(v) and D-IV, 1.4.2);[Rule 76(1); Rule 76(2)(c); ]

(v)if the assumed infringer has appointed a representative, the representative's name and address of place of business in accordance with subparagraph (ii) as set out above (see D‑IV, 1.2.2.2(iii) and D-IV, 1.4.2).[Rule 76(2)(d); ]

D‑IV, 1, sets out further details and explains how to deal with the intervention if one of these requirements is not fulfilled.[Rule 77(1); ]