The writer Nancy Mitford spied on her sisters because of their sympathies with Hitler, documents released by the National Archives reveal.
by Paul Reynolds
BBC News Online world affairs correspondent
The latest batch of files from the security service MI5 includes a report from January 1941 which says the writer "personally informed the authorities of her sister's (Lady Mosley's) treasonable sympathies.
Friend of fascism: Diana, second left, at a Nuremberg Rally in 1936
"The information was given with very good will and is thoroughly reliable."
Lady Diana Mosley married the British blackshirt leader Sir Oswald Mosley and was a friend of Hitler.
A search of her luggage at Croydon airport on one occasion, the files state, turned up a newly signed photograph of him. She was interned in late 1940.
In a file named "Persons considered dangerous by Mrs Peter Rodd" (Nancy Mitford's married name), Diana is said to have expressed "the hope, in front of her governess and eldest son, that the British army would be successfully cut off and destroyed at Dunquerque".
Secret marriage in Berlin
The file gives Nancy's view of Lady Mosley: "She is a ruthless and shrewd egotist, a devoted fascist and admirer of Hitler and sincerely desires the downfall of England and democracy in general."
Another file reveals that MI5 knew about Diana's secret marriage to Mosley not long after it took place in 1936.
It was two years before the marriage was publicly acknowledged.
A report from "our man in Berlin" says that he was told of the marriage by another Mitford sister Unity. It was held in propaganda minister Josef Goebbels' office and attended by Hitler.
Nancy Mitford, whose books include Noblesse Oblige, about the behaviour of the British aristocracy, also informed on her sister Pamela.
She and her husband Derek Jackson "had been heard to declare a) that all Jews in England should be killed and b) that the war should be stopped now 'before we lose any more money'", the files show.
In a further file, Mitford speaks of her sister Unity's attempted suicide.
Unity was also close to Hitler, but Mitford said she had "recanted on the subject of anti-semitism", and that "the motives for her suicide was in a sense patriotic, namely despair at the declaration of war between Germany and England, which Hitler personally promised her would never take place".
Diana Mosley's former father-in-law, Lord Moyne, also joined in what appears to have been a family industry of informing.
He wrote a letter to the authorities saying: "It has been on my conscience for some time... that the authorities concerned are aware of the extraordinarily dangerous character of my former daughter-in-law."
Lady Mosley, he said, "spoke of the destruction of the docks at London and Liverpool and said that by this means it would be possible for England to be starved into submission".
Diana Mosley had divorced Lord Moyne's son Bryan Guinness.
Lord Moyne's letter may have been instrumental in finally persuading the government to intern her.