1975 United Kingdom European Communities membership referendum

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%.

This was the first national referendum ever to be held throughout the United Kingdom but due to the ancient principle of parliamentary sovereignty the result could not be binding on either the Government or Parliament, and would remain the only UK-wide referendum until the 2011 referendum on the Alternative Vote system was held thirty-six years later. It was also the only national referendum to be held on the UK's relationship with Europe until the 2016 referendum on continued EU membership.

The February 1974 general election had yielded a Labour minority government, which went on to win a very small overall majority in the October 1974 general election. Labour pledged in its February 1974 manifesto to renegotiate the terms of British accession to the EC, and then to consult the public on whether Britain should remain in the EC on the new terms, if they were acceptable to the government. The Labour Party had historically feared the consequences of EC membership, such as the large differentials between the high price of food under the Common Agricultural Policy and the low prices prevalent in Commonwealth markets, as well as the loss of both economic sovereignty and the freedom of governments to engage in socialist industrial policies, and party leaders stated their opinion that the Conservatives had negotiated unfavourable terms for Britain.