I offer an empirical study of the preliminary ruling procedure in the case law of the Czech Constitutional Court. I uncover the Constitutional Court’s understanding of its own role within the preliminary ruling procedure. Because the Constitutional Court did not consider European Union law to fall within its framework of reference it created a judicial construction linking the failure to refer with fundamental rights. The Constitutional Court stipulated an additional set of criteria other than CILFIT to assess the potential breach. The preliminary ruling procedure under Art. 267 TFEU is meant to safeguard uniform interpretation of EU law. The Constitutional Court within the constitutional complaint proceedings safeguards the fundamental right of the parties to the proceedings. As a result, the Constitutional Court puts more emphasis on the transparency of the non-referral and the active role of the parties. Thereby it creates additional protection but also responsibility for the party to the proceedings to take an active role. At the same time the Constitutional Court widens the margin of appreciation of the ordinary judges sitting at the apex court by only sanctioning the non-referral that is arbitrary. What is more in some cases it considers the significance of the question for the unity, cohesion and development of EU law, that is, according to the point of view of the Constitutional Court. The Constitutional Court also created additional responsibilities for the judge sitting at the apex court. They need to address the non-referral in a clear and justifiable manner. To assess the potential failure to make a preliminary reference, the Constitutional Court applies its own modified criteria, including procedural and substantive arbitrariness, significance for EU law, and active suggestion of referral by the parties. As a result of the difficult enforcement of the obligation under Art. 267 of the Treaty on the Functioning of the European Union, these criteria directly affect the margin of appreciation of the national judge. I will compare them with CILFIT criteria and show that while some are compatible with the obligation under Art. 267 TFEU, others pose a risk to the unity of European Union law the preliminary reference procedure is meant to protect. I conducted a three-level empirical analysis, including a content analysis of 100 relevant decisions by the Constitutional Court to support these findings.
