Which British prime minister went on to live the longest?

Leonard James Callaghan, Baron Callaghan of Cardiff, KG, (27 March 1912 – 26 March 2005), often known as Jim Callaghan, was a British politician who served as Prime Minister of the United Kingdom from 1976 to 1979 and Leader of the Labour Party from 1976 to 1980. To date, Callaghan remains the only person to have held all four Great Offices of State, having served as Chancellor of the Exchequer (1964–1967), Home Secretary (1967–1970) and Foreign Secretary (1974–1976) prior to his appointment as Prime Minister.

As Prime Minister, he had some successes, but is mainly remembered for the "Winter of Discontent" of 1978–79. During a very cold winter, his battle with trade unions led to immense strikes that seriously inconvenienced the public, leading to his defeat in the polls by Conservative leader Margaret Thatcher.

Callaghan remained Labour Party leader until November 1980, in order to reform the process by which the party elected its leader, before returning to the backbenches where he remained until he was made a life peer as Baron Callaghan of Cardiff. He went on to live longer than any other British prime minister in history at 92 years and 364 days.