The fascist goddaughter of the late king, George V, was sheltered from jail in wartime Britain by MI5, according to British Security Service files.
MI5 documents now reveal Dowager Viscountess Dorothy Downe had her mail intercepted at her home at Hillington near Sandringham but was not interned.
She was noted in official files as a "most fanatical admirer of Hitler" but not involved in pro-Nazi propaganda.
However, unlike some other fascist aristocrats, she avoided jail.
It is believed she tried to get herself locked up alongside her friends Sir Oswald and Lady Diana Mosley by a back door route.
Fear of communism
A 1940 letter to the Times by lawyer, Oswald Hickson, which MI5 believed to have been sent with the approval of the Dowager Viscountess, asked why the British Union parliamentary candidate had not been interned, unlike other supporters.
The newly released file records that Lady Downe was also said to have "for some time almost entirely supported the National Fascists out of her own pocket".
A communication to Sir Alexander Maxwell, then Permanent Under Secretary of State at the Home Office, in September 1940 states the department's reasons, despite her being "undoubtedly" a BU official.
MI5 believed she was not playing an active part in the organisation of the movement, and was not concerned in spreading pro-Nazi propaganda.
She had told Norfolk police of her disgust at Germany's invasion of Norway, Holland and Belgium and had taken down her photographs of Hitler.
The anonymous writer added it was thought she had joined the BU because of a fear of communism, and there were "indications" she was anxious to become a martyr.