European Communities Act 1972 (UK)

The European Communities Act 1972 (c. 68), also known as the ECA 1972, was an act of the Parliament of the United Kingdom which made legal provision for the accession of the United Kingdom as a member state to the three European Communities (EC) – the European Economic Community (EEC, the 'Common Market'), European Atomic Energy Community (Euratom), and the European Coal and Steel Community (ECSC, which became defunct in 2002); the EEC and ECSC subsequently became the European Union.

Source: Wikipedia — European Communities Act 1972 (UK) (CC BY-SA 4.0)