This doctoral thesis aims to determine under what conditions the limitation of the principle of transparency of the public procurement procedure with the right to classified data is legitimate. That is, under what conditions the protection of confidential information and the limitation of the principle of transparency of public procurement do not violate other guaranteed rights of participants in the procedure, in particular the right to a fair trial and the right to an effective legal remedy. Through the conducted research, the legitimacy of the limitation of the principle of transparency is categorized through three fundamental aspects: protection of trade secrets, prevention of distortion of fair market competition, and protection of public interest. The essence of the problem is that the Croatian Public Procurement Act does not regulate the right of access to classified data in the public procurement procedure that a particular business entity has designated as secret based on the law, other regulation, or a general act. Because of this, other business entities who want to file a complaint or lawsuit do not have legal mechanisms to gain insight into the parts of the files that are marked as secret. This situation leads to a conflict between the right to protect the secrecy of confidential data and information and the requirement to respect the principle of transparency in the public procurement procedure. Since the Republic of Croatia does not regulate data confidentiality disputes within the framework of the public procurement procedure, the research conducted a comparative analysis of legislative solutions in all member states of the European Union and the United Kingdom. Based on comparative analysis, proposals for amendments to the Croatian legislation were detected in the legislation of the United Kingdom through the Technology and Construction Court Guide and British case law. Legal and technical mechanisms in the United Kingdom enable participants in the public procurement procedure to access the confidential file documentation they need to protect their procedural rights. On the other hand, those mechanisms ensure the confidentiality of data in the procedure. This is achieved through dealing in rings of confidentiality. The thesis proposes the implementation of the institute of confidentiality rings in the Croatian public procurement procedure. Based on the Croatian Act on the Protection of Undisclosed Information with Market Value the comparative procedure with elements of the described confidentiality rings under British law is in fact already regulated in the Republic of Croatia. Also, the thesis applies the Weight Formula according to Robert Alexy and the result additionally indicates that it is necessary to amend the Croatian public procurement legislation regarding the regulation of the procedure of access to data that is marked as secret in the procedure. The Weight Formula is applied to two conflicting rights - the right to protection of trade secrets and the right to transparency, i.e., equality of arms, fair trial, and effective legal remedy. The thesis proposes amendments to the Croatian Public Procurement Act which are necessary to provide all public procurement procedure participants with fundamental rights to a fair trial and an effective legal remedy. Namely, the thesis introduced the concept of applying the principle of limited transparency through the institute of the confidentiality ring, which is supported by arguments from comparative analysis, the extensive case law of the European Court of Human Rights, the Court of Justice of the European Union, the British Technology and Construction Court, the Italian Regional Court in Lazio, the practice of the Croatian State Commission for the control of public procurement procedures, the High Administrative Court of the Republic of Croatia and by applying the Weight Formula