Cathy Massiter is a British whistleblower and former member of MI5 who revealed that the British security service carried out surveillance of British trade unions, civil rights organisations and the Campaign for Nuclear Disarmament. She sustained her revelations via an affidavit. Massiter also revealed the surveillance of Harriet Harman and Patricia Hewitt, who had respectively held the posts of the legal officer and general secretary of the National Council for Civil Liberties.