The Labour Party is a political party in the United Kingdom that has been described as an alliance of social democrats, democratic socialists and trade unionists. The Labour Party sits on the centre-left of the political spectrum. In all general elections since 1922, Labour has been either the governing party or the Official Opposition. There have been six Labour prime ministers and thirteen Labour ministries.

The party was founded in 1900, having grown out of the trade union movement and socialist parties of the 19th century. It overtook the Liberal Party to become the main opposition to the Conservative Party in the early 1920s, forming two minority governments under Ramsay MacDonald in the 1920s and early 1930s. Labour served in the wartime coalition of 1940–1945, after which Clement Attlee's Labour government established the National Health Service and expanded the welfare state from 1945 to 1951. Under Harold Wilson and James Callaghan, Labour again governed from 1964 to 1970 and 1974 to 1979. In the 1990s, Tony Blair took Labour to the centre as part of his New Labour project which governed under Blair and then Gordon Brown from 1997 to 2010.

The Labour Party currently forms the Official Opposition in the Parliament of the United Kingdom, having won the second-largest number of seats in the 2019 general election. The leader of the party and leader of the opposition is Keir Starmer. Labour is the largest party in the Senedd (Welsh Parliament), being the only party in the current Welsh government. The party is the third-largest in the Scottish Parliament, behind the Scottish National Party and the Scottish Conservatives. Labour is a member of the Party of European Socialists and Progressive Alliance, and holds observer status in the Socialist International. The party includes semi-autonomous London, Scottish, Welsh and Northern Irish branches, however it supports the Social Democratic and Labour Party (SDLP) in Northern Ireland, while still organising there. As of July 2021, Labour has around 430,000 registered members, one of the largest memberships of any party in Europe.