The Guarantee of the Lawful Judge in the Light of the Constitutional Court’s Decision of 25 September 2025

Summary

In its decision of 25 September 2025, the Constitutional Court annulled, as contrary to the Constitution, the phrase “… and this decision shall bind the other court” in Article 166 of the Code of Civil Procedure No. 6100 (HMK). The decision has given rise to intense academic and practical debate, particularly in private-law proceedings, with regard to the principles of binding effect between courts, judicial unity and procedural economy. This study examines the reasoning of the decision from the perspective of constitutional principles, consistency of case law and legal certainty, and assesses the limits of the guarantee of the lawful judge and its effects in practice.

Introduction

By annulling the phrase “… and this decision shall bind the other court” in Article 166 of the HMK, the provision on the joinder of actions, the Constitutional Court’s decision of 25 September 2025 has paved the way for an important debate of principle in Turkish procedural law. Following the publication of the decision, both practitioners and academic circles have put forward differing views on the practical and theoretical consequences that may arise from the annulment of the provision.

The debate has centred on whether the annulled provision was consistent with the principles of procedural economy and judicial unity in proceedings, and whether this binding effect conflicted with the guarantee of the lawful judge. The decision has thus brought onto the agenda not only the constitutional review of a technical provision in the HMK, but also a general theoretical debate on the scope of judicial guarantees.