Costa v ENEL (case 6/64)  ECR 585 - ECJ
.....By contrast with ordinary international treaties, the EEC Treaty has created its own legal system which, on the entry into force of the Treaty, became an integral part of the legal systems of the Member States and which their courts are bound to apply.
By creating a Community of unlimited duration, having its own institutions, its own personality, its own legal capacity and capacity of representation on the international plane and, more particularly, real powers stemming from a limitation of sovereignty or a transfer of powers from the States to the Community, the Member States have limited their sovereign rights, albeit within limited fields, and have thus created a body of law which binds both their nationals and themselves.
The integration into the laws of each Member State of provisions which derive from the Community, and more generally the terms and the spirit of the Treaty, make it impossible for the States, as a corollary, to accord precedence to a unilateral and subsequent measure over a legal system accepted by them on a basis of reciprocity. Such a measure cannot therefore be inconsistent with that legal system. The executive force of Community law cannot vary from one State to another in deference to subsequent domestic laws, without jeopardizing the attainment of the objectives of the Treaty set out in Article 5 (2) and giving rise to the discrimination prohibited by Article 7.
The obligations undertaken under the Treaty establishing the Community would not be unconditional, but merely contingent, if they could be called in question by subsequent legislative acts of the signatories. Wherever the Treaty grants the States the right to act unilaterally, it does this by clear and precise provisions (for example Articles 15, 93 (3), 223, 224 and 225). Applications, by Member States for authority to derogate from the Treaty are subject to a special authorization procedure (for example Articles 8 (4), 17 (4), 25, 26, 73, the third subparagraph of Article 93 (2), and 226) which would lose their purpose if the Member States could renounce their obligations by means of an ordinary law.
The precedence of Community law is confirmed by Article 189, whereby a regulation 'shall be binding and directly applicable in all Member States'. This provision, which is subject to no reservation, would be quite meaning less if a State could unilaterally nullify its effects by means of a legislative measure which could prevail over Community law.
It follows from all these observations that the law stemming from the Treaty, an independent source of law, could not, because of its special and original nature, be overridden by domestic legal provisions, however framed, without being deprived of its character as Community law and without the legal basis of the Community itself being called into question.
The transfer by the States from their domestic legal system to the Community legal system of the rights and obligations arising under the Treaty carries with it a permanent limitation of their sovereign rights, against which a subsequent unilateral act incompatible with the concept of the Community cannot prevail. Consequently Article 177 is to be applied regardless of any domestic law, whenever questions relating to the interpretation of the Treaty arise.