US Edition Michael Straight, who has died aged 87, was the former Soviet spy responsible for telling MI5 that Anthony Blunt - whose lover he had briefly been at Cambridge in the 1930s - was a mole.
When this was publicly revealed soon after the unmasking of Blunt in 1979, Straight, a member of America's super-rich establishment, seemed both flattered and embarrassed by the publicity. In his autobiography, After Long Silence (1993), he claimed to have leaked no official information except for a paper he had written himself.
But six years later the KGB released his file which showed that, using the codename "Nigel", he had sent telegrams, ambassadors' reports and political position papers from the State Department.
Michael Whitney Straight was born on September 1 1916, a son of the New York banker Willard Straight and the heiress Dorothy Whitney; his older brother was the racing driver Whitney Straight, who later became a wartime RAF pilot and vice-chairman of BOAC.
Young Michael went to Lincoln School, New York, then was brought to England by his mother and stepfather, Leonard Elmhirst, who founded the progressive school Dartington Hall; as a 12-year-old pupil, Michael proved an indifferent speller but was familiar with Freud's views on dreams. He travelled in India, took part in a Pittsburgh steel strike and danced in a ballet company sponsored by his mother before going to the LSE and then Trinity College, Cambridge.
Straight was not interested in discussing details of how "the cause" should be advanced; he preferred to demonstrate his socialism by refusing to let his bedder brush his clothes and protesting about the college servants' low wages. After his recruitment into the Party by Blunt, prompted by his distress at the death of the poet John Cornford in the Spanish Civil War, he was passed on to the British Communist James Klugmann and ordered to return to America, although he was in line to become president of the Cambridge Union.
Following a job interview over tea at the White House with President Roosevelt and his wife Eleanor, he obtained an unpaid job in the European division of the State Department. This enabled him to send copies of confidential papers to a contact he only knew as "Michael Green". But he drifted away from Communism after the Soviet Union joined the Allies, and joined the US Army Air Corps, with which he trained as a B-17 pilot, though he never saw action.
When Straight attended a meeting of the Cambridge Apostles at the RAC Club in London after the war, he became embroiled in a row over Czechoslovakia with the historian Eric Hobsbawn, which prompted Burgess to ask if he was now "unfriendly". "If I were, why should I be here?" Straight replied evasively.
By then he had taken over the running of the influential Left-wing journal New Republic, which was subsidised by his mother, and appointed Henry Wallace as editor. Wallace doubled the circulation, then resigned to run for the presidency, declaring that the Marshall Plan would lead to a third world war; but his star faded and, after Straight took over the editorship himself, the magazine endorsed Harry Truman in the 1948 election.
While the United States became increasingly frenzied in its search for Reds under American beds, Straight considered exposing his former associates. According to Straight, he made three attempts to confess, even walking into the British embassy in Washington before his nerve failed; his wife leaked the names of Blunt and Burgess to her psychoanalyst, who felt prevented by a code of conduct from passing them on to the intelligence service. The most convincing of his excuses was that he was afraid of the effect on his young family.
After giving up New Republic Straight concentrated on a literary career. He wrote Trial By Television, an attack on the McCarthyite witch-hunt and a play, Caravaggio. There were also two Western novels.
Finally, when he was offered the chairmanship of the Advisory Council on the Arts by the Kennedy administration in 1963, and a security check was ordered on him, he went to the FBI. Some months later Arthur Martin, the principal MI5 molehunter, attended a lunch given by the FBI's Bill Sullivan where he met Straight, who volunteered to confront Blunt. Although this drove Blunt to admit his guilt, it was felt that evidence exclusively based on the multi-millionaire's word would be unlikely to secure a conviction; so Blunt was offered immunity in return for a full confession. Since nothing was publicly revealed, Straight was still able to become deputy chairman of the National Endowment for the Arts from 1969 to 1977.
He claimed that Blunt had borne him no ill-will when the two men later met (under Martin's supervision). But Blunt refused to meet Straight later when he came to England to publicise his book; he watched him being interviewed on television with distaste.
By then the family link with New Republic had ended, and the journal commented that Straight's "belated confession" had been prompted "only when events in his own career compelled candour".
Straight conceded - in an interview conducted in the plush comfort of one of his homes - that he was "living at an economic level that does not reflect my contribution to American life".
Michael Straight died on January 4. He married, first, Belinda Crompton, then Nina Auchincloss Steers.
In 1998, he married Katharine Gould, who survives him with the five children from the first marriage.