Subject. The principles of subsidiarity proportionality, which serve as the basic principles fordetermining the competence of integration associations, are considered in the article.Aim. The aim of this paper is to analyse the place and the importance of Member States’ obligationsderiving from the EU legal order in order to address the relationships between EU lawand national tax law, as well as to analyse the practice of using of principles of subsidiarity andproportionality by the highest courts of the Russian Federation as a federal state.Methodology. The author uses methods of theoretical analysis, particularly the theory ofintegrative legal consciousness, as well as legal methods, including formal legal method andcomparative law.Results, scope. The exercise of power by the European Union in the areas of shared competencemust respect the principle of subsidiarity. The founding Treaties make clear thatsubsidiarity is a legal enforceable legal principle. However. the case law of the EuropeanCourt of Justice reveals that the enforcement of subsidiarity as a judicial principle has beenineffective.The article examines cross-border loss relief for group companies in the context of EuropeanUnion law and considers how this has affected Member States such as the UK. Thecase law of the Court of Justice is then analysed in an attempt to assess whether some ofthe principles set out in these legislative initiatives found their way to Member State lawsthrough the Court's jurisprudence. Following this, the judicial and legislative response tothe Marks & Spencer judgment in the UK are critically assessed.The practical suggestions are looking at developing EU compatible tax principles to be appliedto cross-border taxation within the EU.Having considered the principles of subsidiarity and proportionality in the context of interactionbetween integration and national tax law, the author suggests directions for improvingthe practice of integration tax law. The supranational judicial authority should dwell onthe position that only those differences that are directly based on the origin of the incomeor nationality of the taxpayer can be justified on the basis of restrictions on fundamentalfreedoms.

Conclusions. The author comes to the conclusion that a co-ordinated approach to crossborder tax is essential. The effectiveness of integration tax law will largely depend on how the ratio of the norms of integration and national law in the tax jurisdiction of the Union and member states will be formed. However, historically direct tax has been viewed by Member States as central to national sovereignty.