Winston Churchill in the Second World War in the context of "Centre Block"

Sir Winston Leonard Spencer Churchill (30 November 1874 – 24 January 1965) was a British statesman, military officer, and writer who was Prime Minister of the United Kingdom from 1940 to 1945 (during the Second World War) and again from 1951 to 1955. For some 62 of the years between 1900 and 1964, he was a Member of Parliament (MP) and represented a total of five constituencies over that time. Ideologically an adherent to economic liberalism and imperialism, he was for most of his career a member of the Conservative Party, which he led from 1940 to 1955. He was a member of the Liberal Party from 1904 to 1924.

Of mixed English and American parentage, Churchill was born in Oxfordshire into the wealthy, aristocratic Spencer family. He joined the British Army in 1895 and saw action in British India, the Mahdist War and the Second Boer War, gaining fame as a war correspondent and writing books about his campaigns. Elected a Conservative MP in 1900, he defected to the Liberals in 1904. In H. H. Asquith's Liberal government, Churchill was president of the Board of Trade and later Home Secretary, championing prison reform and workers' social security. As First Lord of the Admiralty before and during the First World War he oversaw the disastrous naval attack on the Dardanelles (a prelude to the Gallipoli campaign) and was demoted to Chancellor of the Duchy of Lancaster. He resigned in November 1915 and joined the Royal Scots Fusiliers on the Western Front for six months. In 1917, he returned to government under David Lloyd George and served successively as Minister of Munitions, Secretary of State for War, Secretary of State for Air, and Secretary of State for the Colonies, overseeing the Anglo-Irish Treaty and British foreign policy in the Middle East. After two years out of Parliament, he was Chancellor of the Exchequer in Stanley Baldwin's Conservative government, returning sterling in 1925 to the gold standard, depressing the UK economy.

Several academics, Members of Parliament, the general public, and journalists alike have attempted to rank prime ministers of the United Kingdom. Those included below generally consist of only a subset of prime ministers, typically those of the 20th century or those who served after the Second World War.