Harold Wilson was appointed Prime Minister of the United Kingdom by Queen Elizabeth II on 16 October 1964 and formed the first Wilson ministry, a Labour government, which held office with a thin majority between 1964 and 1966. In an attempt to gain a workable majority in the House of Commons, Wilson called a new election for 31 March 1966, after which he formed the second Wilson ministry, a government which held office for four years until 1970.

James Harold Wilson, Baron Wilson of Rievaulx, was a British statesman and Labour Party politician who served as Prime Minister of the United Kingdom twice, from October 1964 to June 1970 and again from March 1974 to April 1976. He was the Leader of the Labour Party from 1963 to 1976, and was a Member of Parliament (MP) from 1945 to 1983. Wilson is the only Labour leader to have formed administrations following four general elections.