The symbiotic action of the EU members proved to be a successful model for preserving general well-being and security. Through the evolutionary process, changing the way of functioning, the European Union has gradually evolved into a powerful integration in transition between an international organization and a transnational community promoting peace, preserving freedom, security and justice without internal frontiers, establishing the Internal Market and promoting sustainable development while promoting economic, social and territorial cohesion. However, the current withdrawal of the United Kingdom from the European Union has created an atmosphere of insecurity and skepticism that has gradually shifted to aversion over the prevailing policy and all levels of European institutions. In this respect, the accession countries of the European Union have been particularly vulnerable, which have begun to re-examine the relevance of the European project in new circumstances and consequently to formulate priorities dictating the pace of the reform of harmonization. In accordance with the above mentioned issue, the purpose of this paper is to examine the challenges of the European Union after the process of breaking up with the United Kingdom. Approach to European integration as a subject of research in this paper has been holistic, and its complexity has undergone qualitative research, starting with defined research goals. Thus, the implications of Brexit on the acceding countries of the European Union were analyzed and the relevance of the current EU model of functioning after Brexit was examined. The results of the research have shown that Brexit is the occasion for a deep analysis of the principles of action of the European Union, which, faced with new challenges, must be adaptive and ready for new changes. At the same time, the results confirmed the need to continue the enlargement of the European Union to the accession countries, regardless of Brexit. The results of this research can contribute to the advancement of existing knowledge and wider understanding of the European integration phenomena and the policy of its expansion at a contemporary moment.