When the Commission considers that the facts found during the investigation are sufficient to justify legal action, it may decide to initiate proceedings. Once the Statement of Objections is served, the procedure becomes fully adversarial. The principle of adversarial proceedings is to allow each party to consider the statement of facts and the legal arguments against it. Pursuant to Article 27 of Regulation No 1/2003, the rights of defense are fully guaranteed to undertakings in the course of the proceedings: they may put forward their point of view on the objections raised by the Commission, have access to the file and be heard. It is therefore essential for the undertaking in question to know with certainty whether or not proceedings have been initiated against it.

On the other hand, prior to the Statement of Objections, the adversarial principle does not apply. The Commission is only required to inform the undertakings of the preliminary investigative measures that specifically concern them. However, the Commission must always hear the undertaking before imposing a fine on the basis of Article 23 or a periodic penalty payment on the basis of Article 24 of Regulation No 1/2003.

However, the rights of the defense must not be irrevocably compromised during the preliminary investigation procedure. Thus, the defendant cannot, under Article 14(3)(g) of the International Covenant on Civil and Political Rights, be compelled to testify against himself or to confess guilt. The Commission therefore may not compel an undertaking to provide it with answers which might involve an admission on its part of the existence of an infringement which it is incumbent upon the Commission to prove.

Nevertheless, an undertaking under investigation cannot claim an absolute right to silence insofar as it has a duty of active cooperation, the non-compliance with which is subject to a financial penalty.