Barbara Anne Castle, Baroness Castle of Blackburn, Baroness Castle was a British Labour Party politician who was a Member of Parliament from 1945 to 1979, making her one of the longest-serving female MPs in British history. 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 was the first woman to have held the office of First Secretary of State.