The aim of this paper is to answer the question of whether the harmonisation of substantive criminal law is a matter of EU competence, which involves expanding the scope of criminalisation by means of use of the blanket authorisation under Art. 83(1) subpara. 3 TFEU means going beyond the principle of conferral. The author, using a dogmatic method and a comparative analysis of Polish and German constitutional law as well as the European integration law, is inclined to provide an affirmative answer.