The Social Democratic Party (SDP) was a centrist to centre-left political party in the United Kingdom. The party supported a mixed economy (favouring a system inspired by the German social market economy), electoral reform, European integration and a decentralised state while rejecting the possibility of trade unions being overly influential within the industrial sphere. The SDP officially advocated "social democracy", but its actual propensity is evaluated as close to social liberalism.The SDP was founded on 26 March 1981 by four senior Labour Party moderates, dubbed the "Gang of Four": Roy Jenkins, David Owen, Bill Rodgers, and Shirley Williams, who issued the Limehouse Declaration. Owen and Rodgers were sitting Labour Members of Parliament (MPs); Jenkins had left Parliament in 1977 to serve as President of the European Commission, while Williams had lost her seat in the 1979 general election. All four had held cabinet experience in the 1970s before Labour lost power in 1979. The four left the Labour Party as a result of the January 1981 Wembley conference which committed the party to unilateral nuclear disarmament and withdrawal from the European Economic Community. They also believed that Labour had become too left-wing, and had been infiltrated at constituency party level by Militant tendency whose views and behaviour they considered to be at odds with the Parliamentary Labour Party and Labour voters.

Shortly after its formation, the SDP formed a political and electoral alliance with the Liberal Party, the SDP–Liberal Alliance, which lasted through the 1983 and 1987 general elections. In 1988, the two parties merged, forming the Social and Liberal Democrats, later renamed the Liberal Democrats, although a minority, led by Owen, left to form a continuing SDP.