Diana Mosley interned after sister revealed she was devoted to Hitler

Diana Mosley was identified to MI5 as a Nazi sympathiser by her former father-in-law and her sister, the novelist Nancy Mitford, classified papers released today show.

The security services already knew that Lady Mosley and the leader of Britain's pre-war fascist party, Sir Oswald Mosley, had married in the presence of Hitler in 1936. But she was only interned at the beginning of the Second World War after a member of the Guinness dynasty, to which she belonged through a brief marriage, reported on her views. Files at the National Archives in Kew, west London, cast a new light on Lady Mosley, one of the glamorous Mitford sisters.

The documents show that such was the pro-Nazi ardour of Lady Mosley, who died this summer in Paris aged 93, that Mitford felt compelled to tell the security services about her. In a memorandum, written under Mitford's married name of Rodd, the author said of her sister: "She is a ruthless and shrewd egotist, a devoted fascist admirer of Hitler (whom, oddly enough, Mosley personally detests) and sincerely desires the downfall of England and democracy generally."

The MI5 file on Lady Mosley, one of dozens kept by the security services on suspected Nazi sympathisers in the higher echelons of British society, details how Mitford, who had become estranged from Lady Mosley, had "personally informed" the authorities of her sister's "treasonable sympathies". It added: "The information was given with very good will and is thoroughly reliable. Mrs Rodd is completely above suspicion."

The Mitford sisters were well-known socialites in London in the 1930s. Lady Mosley, renowned for her beauty which Evelyn Waugh said "ran through the room like a peal of bells", was on speaking terms with figures such as Winston Churchill and John Betjeman.

Her relationship with Hitler was well documented by MI5 agents and informers in the years before the outbreak of war, according to the files. But despite more than 150 pages of surveillance reports, it was not until the security services received a letter from Lord Moyne, head of the Guinness dynasty and father of Lady Mosley's first husband, Bryan Guinness, that the government began to view her as serious threat.

Lady Mosley divorced Guinness in 1934. She was widely known to have taken Mosley as a lover during the marriage and become involved in his British Union of Fascists.

Lord Moyne, who was later assassinated by Jewish extremists in Cairo, appears to have done his own espionage work by using the nanny of his grandson, Lady Mosley's oldest son, to compile a dossier of evidence against her in the absence of any proof held by the security services.

In a letter dated 30 May 1940, Lord Moyne wrote: "It has been on my conscience for some time to (ensure) that the authorities concerned are aware of the extremely dangerous character of my former daughter-in-law."