Documents from a secret Swedish archive have identified “Outcast”, the double agent judged by MI6 as “one of the most successful spies against Germany that the 1939-45 war produced”, as a White Russian émigré previously involved in the most notorious smear in British political history.

The documents reveal him as Alexis Bellegarde, one of four White Russian aristocrats believed to have been behind an infamous forgery 15 years before the war began. The revelations of Bellegarde’s importance to MI6 will increase suspicions that British agents had a hand in the production of the “Zinoviev letter”; its leak to the Daily Mail many believe cost Labour the 1924 general election.

The letter, a message supposedly from Grigory Zinoviev, leader of the Communist International in Moscow, to the Communist party of Great Britain, predicted that a trade deal soon to be concluded between the Labour government and the Soviet Union would open the way to radicalising the British proletariat. It was leaked to the Daily Mail just days before the election, which the Conservatives won resoundingly.

Professor Tore Pryser, an expert on second world war espionage in Scandinavia, said documents obtained by a Swedish researcher, Anders Thunberg, from the archive of Sweden’s immigration department, revealed today in the Observer for the first time, confirmed Outcast’s identity beyond all doubt. “After Thunberg’s research, I am 100% convinced that Outcast and Bellegarde are the same person,” he said.

In MI6: The History of the Secret Intelligence Service, an officially sanctioned history by Professor Keith Jeffery, Outcast is not named but is described as “a Russian émigré, formerly living in Berlin” who “escaped from Tallinn with German help, but at the price of agreeing to work for the Abwehr [German military intelligence] Russian section”.

“When in late 1943 Outcast, ill with tuberculosis, wanted to move with his family from Berlin to Stockholm,” Jeffery adds, “SIS [the Secret Intelligence Service] was supportive, and he managed it early in 1944. But the Swedes refused to let him stay and he was evacuated to Britain, where he died the following year.”

The documents Thunberg found last month show every aspect of the story also to be true of Bellegarde. “The most important thing was the document about the British secret service chief in Stockholm,” he said.

According to records in Sweden’s foreign ministry archives, on 3 February 1944, two weeks before Bellegarde arrived at the Swedish port of Malmö, Cyril Cheshire, the head of MI6’s Stockholm station, told Sweden’s foreign ministry of his impending arrival, promising that the British would take care of him and be able to secure space on a flight to England for the family within two weeks.

“He was afraid that if Bellegarde came to Sweden he would be refused entry,” Thunberg said. “That’s why he contacted them in advance. Only important people could find a seat on a plane over the North Sea to Great Britain.”

Michael Bellegarde, a retired property executive living in Suffolk, Bellegarde’s sole surviving child, said he was also in no doubt that his father was the agent Outcast described in Jeffery’s book: “MI6 just will not, as a matter of the Official Secrets Act, disclose the name, but there are so many similarities.”

He added that his father was already “a tremendous anglophile” at the time of the Russian revolution, having learned to speak and write flawless English in St Petersburg from an Englishman his father had employed as his tutor.

By the time Bellegarde offered his services to MI6’s Helsinki station in September 1940, he had been working for the Abwehr for a little over a month. According to Jeffery’s history, by November Outcast had told the British that “German command was preparing a June campaign against the USSR, which would begin in spring 1941, possibly earlier”. Throughout 1942 and 1943, he provided detailed reports on the impact of RAF bombing campaigns in Berlin and other German cities, and fed MI6 disinformation to the Abwehr.

“By the end of 1942 he had begun to include information on the power struggle between Abwehr and Sicherheitsdienst [the Nazi SS intelligence service], a subject of great interest as well to London, since he belonged to a circle of officers regarded as close to [Abwehr chief] Admiral Canaris,” Jeffery writes. “In early 1943, he was evaluated as ‘the best source of information on the German interior produced so far in the war’.”

According to Jeffery, Outcast “had been an important source on Soviet matters” for MI6 before the war started, raising the possibility that he already had links to British spies in 1924 when, according to his widow, he had a hand in forging the Zinoviev letter.

Irina Bellegarde told the Sunday Times in 1966 that her husband had drafted the letter after being contacted by another émigré, Alexander Gumansky, who told him a request to forge the letter had come from “a person in authority in London”.

The two secured a sheet of official Third International notepaper through a Mr Druzhelovsky, whom Irina Bellegarde described as “a little man, badly in need of some real money”. A fourth man, Edward Friede, whose party trick was forging signatures, then copied Zinoviev’s sign-off.

Supporting Irina Bellegarde’s story is the fact that both Gurmansky and Bellegarde were sentenced to death in absentia by a Soviet court, while Druzhelovsky was captured, put on trial in 1928 and shot.

Pryser goes so far as to suggest that Jeffery kept Bellegarde anonymous in his book to conceal his links to MI6 at the time of the forgery. “Jeffery mentions the names of many officers and agents working for SIS. Why has he forgotten Outcast?” he asks. “Obviously, Jeffery wants to hide that Bellegarde/Outcast worked for SIS on the Zinoviev forgery.”

However, Jeffery denied the affair had affected his decision not to name Outcast. “There were stringent restrictions on naming SIS/MI6 agents, which was only possible if authoritative confirmation existed in publicly available materials,” he wrote. “At the time of writing, no such confirmation was available relating to agent Outcast. The material found by your Scandinavian researchers may constitute such material, but I am not in a position now retrospectively to confirm this. All I can say is that the non-identification of Outcast had nothing to do with the Zinoviev letter affair.”

Michael Bellegarde believes his father forged the letter on his own initiative rather than on London’s orders, aiming to prevent an Anglo-Soviet trade deal.