Assignment of an Undercover Investigator and the Substantive Test: A Review of the Constitutional Court's Decision on Application No. 2021/50710

The aim of uncovering the material truth in criminal proceedings does not mean an unlimited power of inquiry. The obtaining of evidence by lawful methods is accepted as an indispensable element of the rule of law, and this principle is expressly laid down in the sixth paragraph of Article 38 of the Constitution. Within this framework, the procedure of “assignment of an undercover investigator”, resorted to particularly in combating organised and covertly committed offences, is regulated in Article 139 of the Code of Criminal Procedure (CCP, Law No. 5271); however, the limits of this procedure and its relationship with the prohibition on inciting a person to commit an offence have constantly been a subject of debate in practice.

The Constitutional Court's decision on Application No. 2021/50710, decided on 16.12.2025 and published in the Official Gazette of 06.07.2026, makes an important contribution to this debate and demonstrates, on the basis of a concrete case, the circumstances in which investigative activities of law enforcement officers concerning the offence of procuring for prostitution (fuhşa aracılık) violate the right to a fair hearing. The application that is the subject of the decision concerns the allegation that the right to a fair hearing was violated on account of a conviction being based on evidence obtained as a result of provocation by law enforcement.

What Is an Undercover Investigator? Under What Conditions Is One Assigned? (Article 139 CCP)

Article 139 CCP provides that, where there are strong grounds for suspicion based on concrete evidence that the offence under investigation has been committed and evidence cannot be obtained by other means, public officials may be assigned as undercover investigators. Under this provision, such an assignment may be ordered only by a judge; it is further provided that the undercover investigator, while performing his or her duty, may not commit an offence and may not be held responsible for the offences being committed by the organisation to which he or she has been assigned.

Underlying these limitations is the view that, when the State exercises its power to combat crime, directing individuals to commit offences or creating an environment conducive to this is incompatible with the principle of the rule of law.

What Is the Concrete Case Underlying the Decision?

As understood from the application file, within the scope of work conducted on prostitution incidents by police officers serving at the Vice Bureau of the Public Order Branch of the Ankara Provincial Police Department (Ankara Emniyet Müdürlüğü Asayiş Şube Müdürlüğü Ahlak Büro Amirliği), a person named K., about whom information had been received that he was procuring for prostitution, was called as if by a customer. As a result of the conversation, an agreement was reached for two women in exchange for a certain fee; however, it was stated that K. was out of town and that the applicant, who was in Ankara, would contact them and send the women. Indeed, some time later the applicant was called by law enforcement and the two women agreed upon in advance were directed to the front of a hotel.

From this point onwards, the course of events reaches the stage that is critical for the assessment in the decision: law enforcement officers were not content with the applicant merely providing the two women agreed upon in advance; they telephoned the applicant again and asked whether he could find other women, whereupon the applicant stated that he would provide four more women. As a result, a total of six women were directed to the hotel and, without any payment of a fee taking place, the law enforcement officers disclosed their identities and commenced investigative measures.

At the end of the proceedings, the applicant was sentenced, as many times as the number of victims (six times), to two years' imprisonment and a judicial fine of TRY 600, and the deferral of the announcement of the verdict (HAGB) was ordered. As the applicant committed a new offence within the five-year supervision period, the deferred verdict was announced; the appeal (istinaf) was dismissed on the merits by the 6th Criminal Chamber of the Ankara Regional Court of Appeal (Ankara BAM), and the conviction became final.

What Kind of Review Did the Constitutional Court Apply? The Two-Stage Test

The Constitutional Court examined the applicant's allegations from the standpoint of the guarantee of a fair hearing within the scope of the right to a fair trial safeguarded by Article 36 of the Constitution. The decision emphasised that the State's creation, through public officials, of an environment enabling persons presumed likely to commit offences to commit them is incompatible with the principle of the rule of law. Nevertheless, the increase in organised crime and the complex ways in which offences are committed have given rise to the need to develop special investigative techniques, and the use of these techniques does not in itself result in a violation of the right to a fair trial. In assessing allegations of incitement or entrapment, the Constitutional Court adopted a two-stage review.

What Is the Substantive Test?

Under the first stage, the “substantive test”, it is examined whether the actions in the investigation of the officers using undercover investigation techniques were essentially passive. In this assessment, it must be determined whether the officer concerned confined himself or herself to investigating the applicant's activities in a passive manner, or whether he or she exerted an influence such as to enable the commission of the offence. In other words, what is meant by the substantive test is the determination of whether there was incitement or entrapment.

In the concrete case, the Constitutional Court concluded that the law enforcement officers, after contacting the applicant and holding conversations about finding women and giving money, did not confine their role to passive investigation; on the contrary, after the applicant had found two women, they adopted an active stance by asking for four more women. It was established that this active intervention led to an increase in the number of victims and consequently to an increase in the amount of the penalty imposed.

What Is the Procedural Safeguards Test?

As the substantive test yielded a negative result, the “procedural safeguards test” was applied. Under this test, in situations where an allegation of incitement or entrapment has been raised, it is examined whether the necessary steps were taken by the courts. In the concrete case, the Constitutional Court emphasised that the law enforcement officers had conducted the investigative measure on their own initiative, without any judicial instruction, and that no determination had been made as to whether these officers had the status of an undercover investigator or of a law enforcement officer conducting an undercover investigation. Although under Article 139 CCP the assignment of an undercover investigator may be ordered only by decision of a judge, it was shown that in the concrete case there was no such decision of a judge.

It was further noted that at the appeal (istinaf) stage, too, it was not discussed whether the intervention complied with the guarantees of the right to a fair trial or whether the law enforcement officers had acted as an agent provocateur (inciting agent). Within this framework, it was concluded that basing a conviction on evidence obtained as a result of an investigation initiated contrary to the principle of the rule of law is incompatible with a fair hearing as required by Article 36 of the Constitution.

What Is Its Relationship with ECtHR Case Law?

The reasoning of the decision gives broad coverage to the case law of the European Court of Human Rights (ECtHR) on complaints of entrapment. According to the settled case law of the ECtHR, where it is established with sufficient certainty that the national authorities investigated the applicant's activities in an essentially passive manner and did not incite the applicant to commit an offence, the use in criminal proceedings of evidence obtained through the measure of an undercover investigator does not raise an issue under Article 6 of the Convention. By contrast, where, in the absence of any concrete fact indicating that the offence in question could have been committed without the intervention of law enforcement officers, those officers assume an active role, it is concluded that the proceedings were not conducted fairly and that Article 6 of the Convention has been violated. It is seen that the approach adopted by the Constitutional Court in the case that is the subject of the application is largely consistent with the ECtHR standard.

What Does the Dissenting Opinion Say? Why Do We Not Find It Correct?

Some members of the Constitutional Court did not agree with the majority view in the decision. In the dissenting opinion, it was argued that the police officers serving at the Vice Bureau could not be characterised as undercover investigators or inciting agents, and that law enforcement was performing the duties assigned to it within the framework of the Law No. 2559 on the Duties and Powers of the Police (Polis Vazife ve Salâhiyet Kanunu). According to the dissenting members, in the concrete case the police took action upon the existence of a suspicion of an offence, and there is no evidence that the applicant's will was vitiated by error, deceit or threat. Furthermore, with reference to the ECtHR's decision in Eurofinacom v. France, which held that French law enforcement officers' accessing a data communication system as if they were customers on suspicion of procuring for prostitution did not constitute incitement, it was argued that a similar assessment should be made.

In our opinion, the fundamental principle overlooked in the dissenting opinion is the doctrine of “the fruit of the poisonous tree is also poisonous” (fruit of the poisonous tree), one of the universally accepted principles of criminal procedure law. That said, another fundamental issue that must additionally be assessed with respect to the concrete dispute is the distinction between the remote (indirect) and proximate (direct) effect of unlawfully obtained evidence. Indeed, in Turkish criminal procedure law, both in legal scholarship and in judicial practice, there is a predominant view that the remote effect of unlawfully obtained evidence is not accepted. Accordingly, other evidence reached on the basis of evidence that was obtained unlawfully and that cannot be relied upon in the judgment, even if such other evidence was itself obtained by lawful methods, as long as the causal link between it and the initial unlawfulness persists, cannot be relied upon in the judgment. For this reason, we are of the opinion that the approach adopted in the dissenting opinion is incompatible both with the view prevailing in legal scholarship and with settled practice, and in this respect is not legally correct.

What Does the Decision Mean for Practice?

The decision examined clarifies the place of undercover investigation techniques in criminal proceedings along two fundamental axes:

Procedural axis: An assignment falling within the scope of Article 139 CCP may be made only by decision of a judge and only for the offences listed in Article 139(7); for offences outside the catalogue, such as prostitution, the assignment must at least be based on an instruction of the public prosecutor under Articles 160–161 CCP. Activities conducted by law enforcement officers on their own initiative, without either a judge's decision or an instruction of the public prosecutor, remain devoid of legal oversight.

Substantive axis: Where law enforcement, during investigative activity, moves from a passive to an active position and thereby has the effect of expanding the scope of the offence, this renders it unlawful to rely in the judgment on the evidence obtained.

Within this framework, it may be said that the decision constitutes, for both academic circles and practitioners, an important point of reference on how the balance between the power of law enforcement to combat crime and the right of individuals to a fair hearing should be struck. The fact that the decision was criticised by the members in the minority shows that the issue still has controversial aspects in practice and that, in similar cases, the particular circumstances of the concrete case must be assessed meticulously.