The Duty of Sincere Cooperation to the Test of Investigations of the European Public Prosecutor’s Office: Observations on the Judgment EPPO v Court of Auditors (T-99/25)

Introduction

With a judgment rendered on 10 June 2026 in Case T-99/25¹, the General Court of the European Union was called upon to resolve the first inter-institutional dispute involving the European Public Prosecutor’s Office (EPPO). The dispute arose out of the European Court of Auditors’ (ECA) refusal to lift the confidentiality of twelve of its officials, thereby preventing the EPPO from hearing them as witnesses in the framework of a criminal investigation. The case offered an important occasion to test the balance of powers within the Union’s legal order². More specifically, it enabled the General Court to clarify the still largely unexplored relationship between the institutional autonomy of the Union’s bodies and the operational requirements of criminal investigations conducted by the EPPO under Regulation (EU) 2017/1939 (the “EPPO Regulation”)³.

The judgment is of twofold interest. First, it confirms and develops a material conception of the challengeable act by extending it to a letter which, despite neither being formally designated as a decision nor emanating from the statutorily competent body, was found to produce binding legal effects. Second, from an institutional perspective, it strengthens the duty of cooperation with the EPPO, thereby significantly limiting the ability of the Union’s institutions, bodies and agencies to rely on their staff’s duty of confidentiality against the needs of criminal investigations.

Facts of the Case

In an action brought on 10 February 2025, the EPPO asked the General Court to annul the letter of the President of ECA, communicated to the EPPO in December 2024, concerning the EPPO’s request of 26 September 2024 to lift the confidentiality of twelve EU officials to be heard as witnesses in the framework of a criminal investigation. The said investigation stemmed from a report of OLAF regarding suspected irregularities allegedly committed by the then-Chairman, Klaus-Heiner Lehne, and other senior officials of ECA, including in connection with the recruitment of a member of the ECA’ staff.

Starting in early 2023, the EPPO formally requested that ECA waive the immunity enjoyed by the persons under investigation pursuant to Article 11(a) of Protocol No 7 on the privileges and immunities of the EU. Pursuant to the latter provision, officials and other servants of the Union enjoy, within the territory of each Member State and irrespective of their nationality, immunity from legal proceedings in respect of acts performed in their official capacity, including words spoken or written. In his replies to these requests, the President of ECA explained, in essence, that the EPPO had not provided sufficient information to enable it to grant them. He also specified that those replies could not be considered as a refusal to waive the immunity, because such a decision would have to be formally adopted by the college of Members’ of ECA.

Against this backdrop, on 26 September 2024, the European Delegated Prosecutor (EDP) in charge of the file requested, on the basis of Article 19 of the Staff Regulations, that the ECA lift the duty of confidentiality of twelve of its officials so that they could be heard as witnesses. In response, the Head of the ECA Legal Service transmitted, on behalf of the college of Members, a statement which, in substance, refused the EPPO’s request on the grounds that, since the waiver of the immunity of the officials under investigation was insufficiently substantiated, releasing the officials from their duty of confidentiality would therefore be contrary to the interests of the Union and circumvent the absence of waiver of that immunity. That position was communicated to the EPPO by letter of 9 December 2024.

By application lodged on 10 February 2025, the EPPO brought an action for annulment against that position before the General Court. In support of its action, it relied on five pleas in law, namely, first, misuse of powers, second, infringement of Article 13(2) TEU, third, infringement of Article 108(5) of the EPPO Regulation, fourth, infringement of Article 6(1) of the EPPO Regulation and, fifth, in essence, infringement of Protocol No 7 and of the second sentence of the first paragraph of Article 19 of the Staff Regulations.

In turn, the ECA raised two pleas of inadmissibility: first, the absence of a challengeable act, the letter at issue being no more than an informative communication; second, the EPPO’s lack of interest in bringing proceedings, in the absence of any formal and definitive decision of refusal.

A Material Conception of the “Challengeable Act”

In addressing the first plea of inadmissibility, the General Court began its reasoning by recalling the very wording of the first paragraph of Article 263 TFEU which makes an action for annulment available against acts “intended to produce legal effects vis-à-vis third parties”⁴. In line with well-established case-law, such an action is available against any measure, whatever its nature or form, provided it is intended to produce binding legal effects⁵.

Building on the analytical framework recently restated in Swissgrid v Commission⁶, the General Court recalls that assessing whether a measure constitutes a challengeable act must be based on its substance. This requires that its effects must be evaluated against objective criteria, such as the content of the measure, the context of its adoption and the powers of the institution, body or agency from which it emanates. The latter criterion is not to be assessed in the abstract, but as factors capable of shedding light on the concrete analysis of the measure’s content.

In the present case, this well-known analytical approach is applied in a novel inter-institutional setting. The significance of the judgment lies in its extension of that approach to a letter which, formally, presented itself neither as a decision nor as emanating from the statutorily competent body. First, as regards the context, the General Court notes that the contested position constituted a direct response to a specific request submitted on the basis of Article 19 of the Staff Regulations. Second, as to the powers of its author, the General Court considers that the EDP could reasonably regard the position as emanating from the competent authority, given that it had been transmitted “on behalf of” the college of Members of the ECA. Finally, as to the content, the General Court adopts a substantive reading of the measure, considering that, despite its presentation as a mere summary of prior exchanges, its final part amounted, in essence, to a refusal to grant the requested waiver.

This characterisation entails an important consequence. By refusing to waive the duty of confidentiality arising under Article 19 of the Staff Regulations, the ECA prevented the EDP from obtaining testimony from the twelve officials concerned, thereby restricting the exercise of its investigative powers under Article 28(1) of the EPPO Regulation. The letter therefore produced legal effects vis-à-vis the EPPO, notwithstanding that it formally appeared to concern only the legal position of the officials concerned.

The General Court also rejects the ECA’s argument that a measure adopted under Article 19 of the Staff Regulations could only be challenged by the official concerned through an action under Article 270 TFEU. Its reasoning rests here on a logic of effet utile and the need to preserve the effectiveness of judicial review. Accepting the ECA’s argument would have enabled any Union institution, body, office or agency to impede the exercise of EPPO’s investigative powers through a refusal based on Article 19 of the Staff Regulations, without the EPPO having any avenue to contest the legality of that refusal. Such an interpretation would have created a significant gap in the system for protecting the Union’s financial interests established by the EPPO Regulation, offering recalcitrant bodies a legal avenue for non-cooperation. The General Court draws, by analogy, on its judgment in Weddel v Commission⁷, in which the Court of Justice had already accepted the admissibility of an action brought by a company against a Commission refusal to lift an official’s duty of confidentiality so that he could testify before a national court.

The judgment fits therefore within the broader trajectory of case-law that has consistently privileged substance over form in order to preserve the effectiveness of judicial review. It nonetheless presents a particular significance insofar as the extension of the notion of challengeable act was, in this instance, prompted by an institutional applicant. This is all the more noteworthy given that the dispute arose in a context directly affecting the boundaries of the EPPO’s investigative powers and, ultimately, the effective implementation of the EPPO Regulation itself.

Interest in bringing Proceedings and the Preservation of the EPPO’s Investigative Prerogatives

With its second plea of inadmissibility, based on the alleged absence of an interest in bringing proceedings, the ECA contended that the EPPO had allowed the time limit for challenging its refusal to waive the immunity of the persons under investigation to expire. It inferred that such refusal had become definitive and, in consequence, that the EPPO no longer had any interest in gathering evidence, including hearing the twelve officials as witnesses, and therefore no interest in contesting its refusal to lift their duty of confidentiality.

Recalling the case-law in ECB and Commission v Corneli⁸, the General Court observes that an interest in bringing proceedings presupposes that the annulment of the contested act is, of itself, capable of procuring an advantage to the applicant. Accordingly, it notes that no formal decision was, in fact, ever adopted by the college of Members of the ECA on the request to waive immunity. The President of the ECA had himself acknowledged, in several letters, that he lacked that competence. In those circumstances, the General Court dismisses the notion that the President’s earlier letters constituted implied decisions of refusal, since they emanated from an incompetent authority and did not reflect a definitive position of the ECA. It follows that no time limit for bringing proceedings could have started to run against an act that was never adopted in the required form.

In addressing this plea of inadmissibility, the General Court ruled on a significant question of law concerning the conduct of investigations by the EPPO, providing an important clarification as to the scope of Article 39 of the EPPO Regulation. It held that a refusal to waive the immunity of a person under investigation does not, automatically and definitively, preclude the EPPO from pursuing its investigation. Rather, the EPPO remains entitled to seek additional evidence, supplement a pending request, or even submit a new request based on newly emerged factual circumstances, before any decision to discontinue proceedings under Article 39(1)(d). According to the General Court, which carefully distinguished the present case from the findings in cases LR Ģenerālprokuratūra and Kaili v Parliament and EPPO⁹, requiring the prior closure of the investigation to permit the submission of a renewed request to waive immunity would amount to an unnecessary procedural formality, devoid of any functional justification and liable to compromise the effectiveness of the system established to protect the Union’s financial interests. This teleological reading of Article 39 thus confirms that the EPPO enjoys a broad margin of discretion in the conduct of its investigations.

In doing so, the General Court’s reasoning extends beyond the admissibility of the action. By rejecting the ECA’s argument, it clarifies that a refusal to waive immunity or the duty of confidentiality does not exhaust the EPPO’s investigative powers nor definitively determine the course of the investigation. The possibility of supplementing an existing request or submitting a new one in the light of additional evidence remains an inherent feature of the investigative process under the EPPO Regulation.

Such interpretation preserves the procedural autonomy conferred on the EPPO by Regulation 2017/1939. Accepting the ECA’s position would have enabled Union institutions, bodies, offices and agencies, through the refusal of a waiver request, to predetermine the scope and direction of EPPO investigations. Such an outcome would have entailed an undue interference with the investigative powers entrusted to the EPPO by the Regulation and would have risked undermining the effectiveness of the enforcement system established to protect the Union’s financial interests.

A reinforced Duty of Cooperation towards the EPPO

On the merits, the judgment is particularly noteworthy for its interpretation of the notion of “interests of the Union” in the second sentence of the first paragraph of Article 19 of the Staff Regulations. That provision permits an institution to prohibit an official from testifying only where disclosure would prejudice the interests of the Union¹⁰.

The General Court reiterates, in line with its earlier case-law, that this derogation must be interpreted strictly. The interests capable of justifying such a refusal must constitute “interests of considerable importance which are vital to the European Union”. The threshold thus is therefore deliberately high. Measured against that standard, the ECA’ reliance on the need to preserve the integrity of its activities and to avert potential reputational harm proved insufficient. The judgment makes clear that abstract concerns cannot, without more, justify preventing EU officials from cooperating with a criminal investigation conducted by the EPPO. Instead, the institution concerned must demonstrate concretely how the proposed testimony would seriously undermine interests of the Union of exceptional importance. Importantly, the General Court went further still, observing that it is in the very interest of the Union to allow the EPPO to gather evidence throughout its investigation¹¹.

The judgment therefore confirms that, save in genuinely exceptional circumstances, the balance struck by Article 19 is one that favours cooperation with the EPPO rather than institutional secrecy. Although the General Court grounded its reasoning primarily in the interpretation of Article 19 of the Staff Regulations, it borrows its deepest logic from the principle of sincere cooperation, which requires a Union whose bodies loyally contribute to shared objectives¹². This reasoning transforms Article 19 from a provision primarily concerned with protecting institutional confidentiality into a rule capable of shaping interinstitutional relations.

In doing so, the judgment also reinforces a fundamental rule of law imperative within the Union legal order, namely that no Union institution or body is exempt from the requirements of EU law. The credibility of the Union’s commitment to the rule of law therefore depends not only on its ability to require compliance from the Member States, but also on its willingness to ensure that its own institutions are subject to effective oversight where the protection of the Union’s financial interests is at stake¹³.

Beyond its procedural interest, the judgment carries an institutional significance that reaches well beyond the present dispute. By rejecting the ECA’s argument, the General Court implicitly excludes the possibility for a Union body to invoke any form of institutional privilege that would shield from the requirements of EPPO’s criminal investigations. Had such a position been validated, any institution could have neutralised the prerogatives that Regulation 2017/1939 confers on the EPPO for the protection of the Union’s financial interests.

More broadly, this solution fits within a wider movement of construction of a genuine European criminal justice system, of which the EPPO has been the cornerstone since it became operational in 2021. The judgment makes clear that this novel EU architecture cannot be frustrated by institutional resistance, even in good faith, in the course of an investigation. Whether this judgment will ultimately have a lasting impact on the balance between the institutional autonomy of the Union institutions and the effectiveness of the Union’s system of criminal enforcement remains to be seen. Nevertheless, it already provides an important point of reference for the future development of the evolving framework governing cooperation between the EPPO and other Union institutions and bodies¹⁴.

^* The opinions expressed are those of the author only and do not necessarily reflect those of the Court of Justice of the European Union.

¹ General Court, 10 June 2026, Case T-99/25, European Public Prosecutor’s Office v European Court of Auditors.

² M. Caianiello, Immunity and Sincere Cooperation: When the EPPO Challenges the Court of Auditors, in European Journal of Crime, Criminal Law And Criminal Justice, 2025, pp. 313-325.

³ Council Regulation (EU) 2017/1939 of 12 October 2017 implementing enhanced cooperation on the establishment of the European Public Prosecutor’s Office (‘the EPPO’), in OJEU, L 283 of 31 October 2017, pp. 1-71.

⁴ K. Lenaerts, K. Gutman, J.T. Nowak, EU Procedural Law, Oxford University Press, 2023; A. H. Türk, Judicial Review in the European Union, Edward Elgar Publishing, 2025.

⁵ See Court of Justice, 11 November 1981, Case 60/81, IBM v Commission.

⁶ General Court, 13 February 2025, Case T-121/23 P, Swissgrid v Commission.

⁷ Court of Justice, 18 February 1992, Case C-54/90, Weddel v Commission.

⁸ Court of Justice, 15 July 2025, Joined Cases C-777/22 P and C-789/22 P, ECB and Commission v Corneli.

⁹ Court of Justice, 30 November 2021, Case C-3/20, LR Ģenerālprokuratūra and General Court, 16 January 2024, Case T-46/23, Kaili v Parliament and EPPO. In particular, the former held that immunity does not preclude investigative measures or the gathering of evidence, but limits the use of such evidence to try and convict the person concerned for acts covered by that immunity, whereas the latter characterised a request to lift immunity as a preliminary and necessary measure intended to ensure the effectiveness of the investigations where immunity constitutes an obstacle thereto.

¹⁰ T. Livolsi, R. Schiano, Droits et obligations du fonctionnaire (art. 11 à 26 bis), in V. Giacobbo Peyronnel, E. Perillo (dirs.), Statut de la fonction publique de l’Union européenne, Bruxelles, 2017, pp. 161-186.

¹¹ General Court, European Public Prosecutor’s Office v European Court of Auditors, cit., para. 69.

¹² F. Casolari, Leale cooperazione tra stati membri e Unione europea, Napoli, 2020.

¹³ L. Marin, ‘The Importance of Being Earnest?’ The Fight Against Fraud within the Institutions of the Union, in Rivista del Contenzioso Europeo, 2025, p. 1 ff.

¹⁴ A. Weyemberg, C. Brière, The inter-agency cooperation and future architecture of the EU criminal justice and law enforcement area, Study for the LIBE Committee, European Parliamentary Research Service, 2014.