Accession of the United Kingdom to the European Communities in the context of "European Union Association Agreement"

The 2016 United Kingdom European Union membership referendum, commonly referred to as the EU referendum or the Brexit referendum, was a referendum that took place on 23 June 2016 in the United Kingdom (UK) and Gibraltar under the provisions of the European Union Referendum Act 2015 to ask the electorate whether the country should continue to remain a member of, or leave, the European Union (EU). The result was a vote in favour of leaving the EU, triggering calls to begin the process of the country's withdrawal from the EU commonly termed "Brexit".

Since 1973, the UK had been a member state of the EU and its predecessor the European Communities (principally the European Economic Community), along with other international bodies. The constitutional implications of membership for the UK became a topic of debate domestically particularly regarding sovereignty. A referendum on continued membership of the European Communities (EC) to try and settle the issue was held in 1975, with 67% of voters approving continued membership. Between 1975 and 2016 as European integration deepened, subsequent EC/EU treaties and agreements were ratified by the UK Parliament but without any public approval. Following the Conservative Party's victory at the 2015 general election as a main manifesto pledge, the legal basis for the EU referendum was established through the European Union Referendum Act 2015. Prime Minister David Cameron also oversaw a renegotiation of the terms of EU membership, intending to implement these changes in the event of a Remain result. The referendum was legally non-binding due to the ancient principle of parliamentary sovereignty, although the government promised to implement the result.

Since 1945, immigration to the United Kingdom, controlled by British immigration law and to an extent by British nationality law, has been significant, in particular from the former territories of the British Empire and the European Union.

Since the accession of the United Kingdom to the European Communities in the 1970s and the creation of the EU in the early 1990s, immigrants relocated from member states of the European Union, exercising one of the European Union's Four Freedoms. Immigration to and from Central and Eastern Europe has increased since 2004 with the accession to the European Union of eight Central and Eastern European states, since there is free movement of labour within the EU. In 2021, since Brexit came into effect, previous EU citizenship's right to newly move to and reside in the UK on a permanent basis does not apply anymore.

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.

The Act also incorporated Community Law (later European Union Law), along with its acquis communautaire, its treaties, regulations, directives, decisions, the Community Customs Union (later European Union Customs Union), the Common Agricultural Policy (CAP), the Common Fisheries Policy (FCP) together with judgments of the European Court of Justice into the domestic law of the United Kingdom.

The 1975 United Kingdom European Communities membership referendum, also known variously as the Referendum on the European Community (Common Market), the Common Market referendum and EEC membership referendum, was a non-binding referendum that took place on 5 June 1975 in the United Kingdom (UK) under the provisions of the Referendum Act 1975 to ask the electorate whether the country should continue to remain a member of, or leave, the European Communities (EC) also known at the time as the Common Market — which it had joined as a member state two-and-a-half years earlier on 1 January 1973 under the Conservative government of Edward Heath. The Labour Party's manifesto for the October 1974 general election had promised that the people would decide through the ballot box whether to remain in the EC.

In a 1975 pamphlet, Prime Minister Harold Wilson said: "I ask you to use your vote. For it is your vote that will now decide. The Government will accept your verdict." The pamphlet also said: "Now the time has come for you to decide. The Government will accept your decision — whichever way it goes." The electorate expressed significant support for continued EC membership, with 67% in favour on a national turnout of 64%.