The 1945 United Kingdom general election was a national election held on 5 July 1945, but polling in some constituencies was delayed by some days, and the counting of votes was delayed until 26 July to provide time for overseas votes to be brought to Britain. The governing Conservative Party sought to maintain its position in Parliament but faced challenges from public opinion about the future of the United Kingdom in the post-war period. British Prime Minister Winston Churchill proposed to call for a general election in Parliament, which passed with a majority vote less than two months after the conclusion of the Second World War in Europe.The election's campaigning was focused on leadership of the country and its postwar future. Churchill sought to use his wartime popularity as part of his campaign to keep the Conservatives in power after a wartime coalition had been in place since 1940 with the other political parties, but he faced questions from public opinion surrounding the Conservatives' actions in the 1930s and his ability to handle domestic issues unrelated to warfare. Clement Attlee, leader of the Labour Party, had been Deputy Prime Minister in the wartime coalition in 1940-1945 and was seen as a more competent leader by voters, particularly those who feared a return to the levels of unemployment in the 1930s and sought a strong figurehead in British politics to lead the postwar rebuilding of the country. Opinion polls when the election was called showed strong approval ratings for Churchill, but Labour had gradually gained support for months prior to the war's conclusion.

The final result of the election showed Labour to have won a landslide victory, making a net gain of 239 seats, winning 47.7% of the popular vote and achieving a majority of 145 seats, thus allowing Attlee to be appointed prime minister. This election marked the first time that the Labour Party had won an outright majority in parliament, and allowed Attlee to begin implementing the party's post-war reforms for the country. For the Conservatives, the Labour victory was a shock, as they suffered a net loss of 189 seats although they won 36.2% of the vote and had campaigned on the mistaken belief that Churchill would win as people praised his progression of the war. Of the other two major parties, the Liberal Party faced a serious blow after taking a net loss of nine seats with a vote share of 9.0%, many within urban areas and including the seat held by its leader, Archibald Sinclair. The Liberal National Party fared significantly worse, enduring a net loss of 22 seats with a vote share of 2.9%, with its leader Ernest Brown losing his seat.

The 10.7% swing from the Conservatives to an opposition party is the largest since the Acts of Union 1800; the Conservative loss of the vote exceeded that of the 1906 Liberal landslide ousting of a Conservative administration. It was also the first election since 1906 in which the Conservatives did not win the popular vote. Churchill remained actively involved in politics and returned as prime minister after leading his party into the 1951 general election. For the National Liberals, the election was their last as a distinct party, as they merged with the Conservatives in 1947 while Ernest Brown resigned from politics in the aftermath of the election.

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.