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.