The Social Democratic Party (SDP) was a political party of the United Kingdom that existed nationwide between 1981 and 1988. It was founded by four senior members of the right-wing of the Labour Party, dubbed the "Gang of Four": Roy Jenkins, David Owen, Bill Rodgers and Shirley Williams. At the time of the SDP's creation, 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. The four left the Labour Party in the belief that it had become too left-wing, and had been infiltrated at constituency level by Trotskyist factions whose views and behaviour were at odds with the parliamentary party and the Labour-voting electorate.

For the 1983 and 1987 General Elections, the SDP joined the Liberal Party in the SDP-Liberal Alliance. The party merged with the Liberal Party in 1988 to form the Liberal Democrats, although a minority left to form a continuing SDP.