The aim of the publication is to establish the function of the legal regulation of the principle of resolving legal doubts in favour of the citizen (in dubio pro libertate), introduced by the amendment of 2017 to the Code of Administrative Procedure. The purpose of the article is also to compare the state of the regulation in force at the time of its entry into force with the legal state created after the introduction of the amendment. It has been argued that the introduced principle worsened the legal situation of citizens in relation to their situation before its entry into force. The paper briefly presents the functions of law, including procedural law. This principle has been discussed and it has been established that it has a protective function. First and foremost, to ensure the security and stability of the law when the legislator has failed to fulfil its duty to legislate correctly and fairly. This function is of particular importance in cases where an administrative body unilaterally determines constitutional rights and freedoms. Resolving legal doubts in favour of the citizens is of particular importance in administrative law, which is very dynamic area (inflation and atomisation of this law), specialised (e.g. technical conditions of development) and complex due to the existence of various hierarchies of sources of law (constitution, statutes and secondary legislation, including local laws). The principle also protects other values, such as an important public interest. As a result of the comparison of the state of regulation before 2017, it was established that as long as the principle of in dubio pro libertate was a postulate of the doctrine, court judgments, interpreted from other principles of administrative proceedings, it did not suffer such significant limitations and could serve more fully to protect an entity. It the current state of law, administrative bodies can make use of it in a limited way only to several proceedings. Nor does it apply if there is – as is common in administrative proceedings – an important social interest or conflicting legal or factual interests of the parties concerned. The originality of the work results from taking into account the case law developed since the entry into force of the principle and the comparison and analysis of the situation of citizens before and after its introduction into the Code. The research carried out for the purpose of this work will be based on the comparative legal method as well as legal-dogmatic analysis, including the analysis of Polish normative material and the resulting views of the judicature and doctrine of law.