The adversarial principle consists in allowing each of the parties to contest the statement of facts and the legal grounds brought against them. Compliance with this principle must be ensured during the investigation and the proceedings before the Competition Authority, subject to the provisions of Article L. 463-4 relating to business secrecy.

The administrative investigation is not subject to the adversarial principle since the gathering of evidence of alleged infringements does not prejudge the culpability of the undertakings. Moreover, whether the operations are carried out within the framework of Article L. 450-3 (basic investigation) or Article L. 450-4 ( in-depth /court ordered investigation), the formal requirements imposed by the Commercial Code tend to guarantee the rights of the undertakings under investigation, in particular by excluding from the file or the proceedings any documents or materials that have been improperly obtained. For the adversarial principle to be considered as having been complied with during the preliminary investigation, it is sufficient for the parties to be in a position to submit observations in good time at the Statement of Objections and the report, as well as before the Competition Authority. The fact that they were not heard during the investigation phase does not affect the proper conduct of the proceedings..

On the other hand, under Article L. 463-1 of the Commercial Code, the investigation and the proceedings before the Competition Authority are fully contradictory. The Statement of Objections opens the investigation before the Competition Authority and the adversarial procedure. The addressees of the objections may, from that moment on, consult the file and present their observations within two months. It is not necessary, at this stage of the procedure, to disclose the documents on which the rapporteur has relied, since the adversarial nature of the procedure is guaranteed by the possibility of consulting the entire file at the headquarters of the Competition Authority.

The addressees of the objections have a period of two months to submit their observations in response to the Statement of Objections. As the starting point of the two-month period is not imposed subject to nullity, it may be postponed in order to preserve the principle of adversarial proceedings and the rights of the parties. Since the observations of the Government Commissioner do not constitute objections, they do not give rise to a new time-limit for reply. The parties have a period of fifteen days to take cognizance of them. An additional period of one month can be granted by the President of the Authority in exceptional circumstances.

Finally, when expert witnesses are involved, the parties must be called upon to participate in the operations and be able to make their observations, otherwise the adversarial principle would not be observed. Similarly, respect for the adversarial principle and the rights of the defense implies that the parties or their representatives must be able to know the names and status of the witnesses that the Authority intends to hear before the hearings.

In the event of an appeal before the Paris Court of Appeal, observance of the adversarial principle requires the Court to ensure and indicate in its decision that the parties have been given the opportunity to reply to the Authority’s written observations. It cannot rule on a decision of inadmissibility without hearing or informing the persons concerned by the complaint.