Leonard James Callaghan , Baron Callaghan of Cardiff , KG , PC , ( 27 March 1912 â `` 26 March 2005 ) , was Labor Prime Minister of the United Kingdom from 1976 to 1979 . Leonard James Callaghan , Baron Callaghan of Cardiff , KG , PC ( 27 March 1912 â `` 26 March 2005 ) , was a British Labor politician , who was Prime Minister of the United Kingdom from 1976 to 1979 and Leader of the Labor Party from 1976 to 1980 .