premise: Margaret Thatcher was the earliest woman to lead a major political party in the UK.
hypothesis: Margaret Hilda Thatcher , Baroness Thatcher , ( ; 13 October 1925 -- 8 April 2013 ) was a British stateswoman , who served as Prime Minister of the United Kingdom from 1979 to 1990 and as Leader of the Conservative Party from 1975 to 1990 . In 1975 , Thatcher defeated Heath in the Conservative Party leadership election to become Leader of the Opposition and became the first woman to lead a major political party in the United Kingdom . Second Shadow Cabinet of Edward Heath . She won , becoming the first female leader of a major political party in Britain .