Some insights into the Plea of illegality in French public law, by Clementine Mazille and Olivier Dubos


The validity of a French administrative act may be challenged through a direct challenge, but also by arguing a “plea of illegality”, in the context of a dispute that does not involve the annulment of that act. Thus, the plea of illegality is based on a specific link between two administrative acts and a risk of transmission of illegalities across administrative acts. It supposes first, to rely on irregularities in respect of an earlier administrative act which would have conditioned the enactment of a second measure (which is the subject of the main action), then draw the legal conclusions therefrom in the main action (in annulment or compensation). Since the acceptance of the plea of illegality (Conseil d’Etat, 24 January 1902, Avezard, n°106), the mechanism has evolved, especially over the last few years. Two specific points can be noticed, considering a new balance between the requirements of legality and legal certainty but also the sound administration of justice.

The reduction of the admissibility of the plea

Direct and indirect controls are related to a different balance between legality and legal certainty. Since its acceptance by the case law at the beginning of the 20th century, the plea of illegality contributes to the effectiveness of the requirement of legality. Today, however, this mechanism tends to be more closely supervised because of the importance of a second requirement: the principle of legal certainty. This evolution can be observed through the conditions of a plea of illegality, which tend to be reinforced.

First of all, the link between the act in which illegality is alleged in the main action and the act subject to the exception of illegality has to be narrow to consider that some irregularities of one could affect the other one. Such a link of dependence is frequently admitted in urban planning matters. This requirement has been reinforced (CE, 11 July 2011, Sodemel, n°320735).

Secondly, whereas the mechanism of the plea can be used against a regulatory act, even a definitive one, the legislator and the courts have limited the grounds that can be invoked in such an incidental control. Indeed, the Conseil d’Etat extended a specific solution established since a 1994 law in urban matters to all the hypotheses raising a plea of illegality of a regulatory act. This important case law is even more restrictive because the supreme administrative court now considers that formal and procedural defects “cannot be usefully invoked” after the expiry of the period for contentious appeal, which is in principle two months since the adoption of the act (CE, 18 May 2018, Fédération des finances et affaires économiques de la CFDT, n°414583).

The ‘limitation’ of the “privilege de jurisdiction”

The French duality of orders of jurisdiction results from a historic and specific interpretation of the principle of separation of administrative and judicial authorities. In principle, the administrative courts are the only ones competent to assess the legality of administrative acts, more specifically “the annulment or reversal of decisions taken in the exercise of public authority” by public persons or bodies under their control.

Sometimes, the plea of illegality of an administrative act can also be raised before the judicial courts, however. The competence of criminal courts is recognised “when the solution of the criminal proceedings submitted to them depends on this examination” (Article 111-5 of the Penal Code). In contrast, civil courts have to submit to the competent administrative court for a preliminary ruling. However, in order to reduce the complexity and duration of the procedure, the competence of civil courts has been admitted “when it is clear, in the light of established case law, that the challenge can be accepted by the court hearing the main action” (Trib. Confl., 17 October 2011, SCEA du Chéneau, n°C3828-3829, cons. 8). Also, to respect the principle of primacy of EU law, the civil court is able “in the event of difficulty in interpreting these rules, to refer the matter itself to the Court of Justice for a preliminary ruling or, when it considers itself in a position to do so, to apply Union law” (id.).

To conclude, and despite the reduction of the effectiveness of the incident control of administrative acts in order to reinforce legal stability, the mechanism appears to become a more commonplace plea since the privilege of administrative courts is reduced, and is used daily before French courts. When these courts consider the ground founded, the basic act which had been declared illegal in this incident control is not necessarily repealed but the statement of its irregularity leads to ‘paralyse’ its effects in the main proceedings (the second act contested in the main proceedings is deprived of a legal basis) or to draw the consequences recognising the administration’s liability. However, it is not certain that all the consequences of the recent evolution have been made in particular considering the effects of the plea (which are again specific in urban matters), and the legal system of the plea of illegality may not be fully stabilized as yet.

Posted by Dr Clementine Mazille (UPPA – Bayonne, CDRE) and Professor Olivier Dubos (Univ. de Bordeaux, CRDEI)

Suggested citation: C. Mazille and O. Dubos, “Some insights into the Plea of illegality in French public law”, available at