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INTRODUCTION. The article presents the evolution of legal regulation of the institution of enhanced cooperation as a legal form of flexible integration in European Union (EU) law. Flexible integration is a fairly widespread and objectively existing legal phenomenon in the practice of various integration associations. However, it is within the framework of EU law that the concept of enhanced cooperation has received the greatest development and legal regulation. This was largely due to the EU's desire to regularize and subordinate alternative forms of co natural that interstate cooperation within the framework of one or another integration cooperation acquires more and more complex character, and cooperation of the member states of an integration association within the framework of one or another direction of interaction has uneven and heterogeneous character. MATERIALS AND METHODS. The methodological basis of the study was traditional general scientific and special methods of cognition of legal phenomena: comparative legal method; method of scientific analysis; formal legal method; method of synthesis of social and legal phenomena. RESEARCH RESULTS . As a result of the analysis, the main stages of development of prerequisites and reasons for the legal regulation of enhanced cooperation have been identified and the periodization of the development of legal regulation of this institution has been proposed. DISCUSSION AND CONCLUSIONS . Within the framework of the proposals on the periodization of the development of legal regulation of the institution of enhanced cooperation the following stages can be distinguished. The first stage can be labeled as “pre-Amsterdam”. This period is characterized by the formation of conceptual foundations of enhanced cooperation as a legal form of flexible integration. Within the framework of the “pre-Amsterdam” period there is an application of some form of flexible integration (Schengen, euro zone, etc.). The conceptualization of enhanced cooperation as a form of flexible integration was the basis for the normative-legal consolidation of this institution in the founding treaties of the EU. The next stage can be called “Amsterdam-Nicean”. Although the institution of enhanced cooperation received different names in the Amsterdam and Nice versions of the establishing treaties, it seems possible to unite this period into one: despite the normative-legal consolidation of enhanced cooperation as a form of flexible integration, the relevant norms of the establishing treaties have not actually received practical application. Finally, the next stage – the “Lisbon” period is characterized by the active ap1plication of the norms of the establishing treaties in the EU practice, including judicial.
Войников et al. (Mon,) studied this question.