The Liberal Party was one of the two major political parties in the United Kingdom, along with the Conservative Party, in the 19th and early 20th centuries. Beginning as an alliance of Whigs, free trade–supporting Peelites and reformist Radicals in the 1850s, by the end of the 19th century it had formed four governments under William Gladstone. Despite being divided over the issue of Irish Home Rule, the party returned to government in 1905 and won a landslide victory in the 1906 general election.

Under prime ministers Henry Campbell-Bannerman (1905–1908) and H. H. Asquith (1908–1916), the Liberal Party passed reforms that created a basic welfare state. Although Asquith was the party leader, its dominant figure was David Lloyd George. Asquith was overwhelmed by the wartime role of coalition prime minister and Lloyd George replaced him in late 1916, but Asquith remained as Liberal Party leader. The split between Lloyd George's breakaway faction and Asquith's official Liberal Party badly weakened the party.

The coalition government of Lloyd George was increasingly dominated by the Conservative Party, which finally deposed him in 1922. By the end of the 1920s, the Labour Party had replaced the Liberals as the Conservatives' main rival. The Liberal Party went into decline after 1918 and by the 1950s won as few as six seats at general elections. Apart from notable by-election victories, its fortunes did not improve significantly until it formed the SDP–Liberal Alliance with the newly formed Social Democratic Party (SDP) in 1981. At the 1983 general election, the Alliance won over a quarter of the vote, but only 23 of the 650 seats it contested. At the 1987 general election, its share of the vote fell below 23% and the Liberals and the SDP merged in 1988 to form the Social and Liberal Democrats (SLD), who the following year were renamed the Liberal Democrats. A splinter group reconstituted the Liberal Party in 1989.

Prominent intellectuals associated with the Liberal Party include the philosopher John Stuart Mill, the economist John Maynard Keynes and social planner William Beveridge. Winston Churchill authored Liberalism and the Social Problem (1909), praised by Henry William Massingham as "an impressive and convincing argument" and widely considered as the movement’s bible.