Barbara Anne Castle, Baroness Castle of Blackburn, (' Betts'"; 6 October 1910 – 3 May 2002), was a British Labour Party politician who was a Member of Parliament from 1945 to 1979, making her the second longest-serving female MP after Gwyneth Dunwoody. Regarded as one of the most significant Labour Party politicians, Castle developed a close political partnership with Prime Minister Harold Wilson and held several roles in the Cabinet. She remains to date the only woman to have held the office of First Secretary of State.