The position with regard to legislative competence is quite clear. In circumstances where the EU Treaty does not assign legislative competence to the EU, then competence remains vested in Member States. The EU Treaty makes no explicit provision for legislative competences in the area of direct taxation.
EU legislation on the taxation of companies is therefore based on Article 115 of the Treaty on the Functioning of the EU, which authorises the Union to adopt directives which directly affect the internal market. Any Directives made under Article 115 require unanimity among all Member States.
Therefore a Member State retains sovereignty over any tax area not governed by a Directive. Most importantly Member States define their own tax bases and retain the right to bilateral approaches on tax matters.