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Mandatory Credit: Photo by Neville Marriner/Daily Mail/REX (1146790a) Margaret Thatcher Leaving Downing St As She Resigns. Former Prime Minister Margaret Thatcher Leaves Number 10 Downing Street For The Last Time After She Was Defeated By John Major In The Conservative Party Leadership Election Of 1990. 30/11/1990 Margaret Thatcher Leaving Downing St As She Resigns. Former Prime Minister Margaret Thatcher Leaves Number 10 Downing Street For The Last Time After She Was Defeated By John Major In The Conservative Party Leadership Election Of 1990. 30/11/1990