The United Kingdom holds its first country-wide referendum on membership of the European Economic Community (EEC).

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.

The European Economic Community (EEC) was a regional organisation created by the Treaty of Rome of 1957, aiming to foster economic integration among its member states. It was subsequently renamed the European Community (EC) upon becoming integrated into the first pillar of the newly formed European Union (EU) in 1993. In the popular language, the singular European Community was sometimes inaccurately used in the wider sense of the plural European Communities, in spite of the latter designation covering all the three constituent entities of the first pillar. The EEC was also known as the European Common Market (ECM) in the English-speaking countries, and sometimes referred to as the European Community even before it was officially renamed as such in 1993. In 2009, the EC formally ceased to exist and its institutions were directly absorbed by the EU. This made the Union the formal successor institution of the Community.

June 5 is the 156th day of the year in the Gregorian calendar; 209 days remain until the end of the year.