There was one question on the lips of Marxist intellectuals of a certain age yesterday, on hearing of a list of "crypto-communists" provided to the British secret services by the socialist author George Orwell in 1949: "Am I on it?"

A selection of yesterday's vanguard, all now dead - including Cecil Day-Lewis, George Bernard Shaw, Michael Redgrave and Orson Welles - are almost certainly on it. Christopher Hill, the Marxist historian - very much alive - is sure he's on it. And Orwell's former editor at the Observer, David Astor, is certain he won't be on it.

But the real answer was known only to Orwell and former members of the Foreign Office's covert propaganda unit, the Information Research Department, to whom the author agreed to supply a list of "fellow travellers ... not to be trusted". A private list of more than 100 names kept by Orwell to "sort [people] out and determine which of them is honest" will remain under lock and key at Orwell's executors "until all the people named on it are dead". The list he made for the IRD in 1949, meanwhile, is impounded by the Foreign Office.

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The existence of the IRD names, apparently transcribed from Orwell's private notebook and, in the novelist's words, "a list of journalists and writers who in my opinion are crypto-communists, fellow travellers or inclined that way, and should not be trusted", emerged last week through documents released at the Public Record Office under the 30-year rule.

So do socialists feel betrayed by the vision of a desperately sick Orwell, naming names to the authorities from his Gloucestershire sanatorium?

Tony Benn, the Labour MP, regrets that Orwell "gave in", but recalls that "lots of good people went that way". "You have to remember that the biggest terrorists in the world are states," he said.

Three years earlier, in 1946, the IRD had invited Mr Benn to join them. "They wanted journalists and MPs and all sorts of people," he recalls. "People don't realise how much MI5 and MI6 are actually like the CIA and the KGB."

While working at the BBC, Mr Benn was offered between pounds 490 and pounds 1,100 per year to "come on board", he said. "I told them I was considering becoming an MP and they said that was perfectly all right. They had also infiltrated Labour Party HQ and the trade union movement. Years ago we were thought to be paranoid, but it turns out we were quite right."

If Mr Benn is not bitter about Orwell's collusion with the secret Foreign Office propaganda unit, however, Christopher Hill laments the spectacle of a socialist betraying his own side.

"I always knew he was two-faced," says Professor Hill. "There was something fishy about Orwell. I am pained and sorry to hear of it and it confirms my worst suspicions about the man. It is consistent with the general tone of his stories and his journalism, which was always very ambiguous.

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"Animal Farm is precisely an attack on communism."

To David Astor, editor of the Observer from 1949 to 1975 and a close friend of Orwell's, however, the author of Animal Farm and Nineteen Eighty- Four was simply trying to defend his ideals. "Orwell wasn't betraying the left - the pro-communists were betraying us," he said.

"The British intellectual left in those days thought communism was fine, the proper side to be on. They didn't make a distinction between the social democrats and the communists. But at that time communism was a rampant doctrine which was very aggressive." Orwell had seen communism up close in the Thirties, when he fought against Franco in the Spanish civil war, he said. "That gave him a chance to examine the character of the communists. He was in an anarchist regiment and they didn't like anarchists - he came within danger of his life. Animal Farm was about how Marxism produces a regime indistinguishable from fascism."

Mr Astor is upset by claims that Orwell betrayed his own kind, stressing that the purpose of the list was not to have the suspected "crypto-communists" locked up, but to help the IRD identify suitable leftists to help them with anti-communist propaganda. "He was not giving them a blacklist," he says. "He was just telling the IRD who not to employ."

To Neal Ascherson, the Independent on Sunday journalist, who worked under Astor for over a decade, Orwell's betrayal was to leap from decrying communism to naming communists to the authorities - whatever the purported purpose. He remembers being taken to a mysterious meeting with a "Foreign Office" official in the 1960s by an Observer journalist while he was the newspaper's soviet affairs correspondent.

"I realise now that the official was probably from the IRD," he says. "I believe they were checking me out to see if I was suitable to be given leaked information which was actually propaganda." Mr Ascherson declined the offer of a special relationship with the Foreign Office, but said that rather ineffective propaganda still made its way to the Observer foreign desk. "I would see these unmarked brown envelopes lying around," he recalled. "They contained material which was always anonymous. This would be very clumsy propaganda, which usually arrived after the story was long dead."

The offer was tempting for Orwell because he wanted to oppose totalitarianism in all its forms, he said, but "there is a difference between being determined to expose the stupidity of Stalinism and the scale of the purges and throwing yourself into the business of denouncing people you know." Paul Foot, the left-wing journalist, sees the list as the next step for a man who "famously believed that Russia was not communist enough". In his early years Orwell was more pro-socialist than anti-communist, but by later life his loathing of anti-democratic communism came to dominate, said Mr Foot.

"He saw the stranglehold the communists had on the left, and he was one of the first people who saw the way Russia was going. He wanted to do something for his country."

The latest revelations about Orwell do not detract from his stature as a great writer, he said. "It is a pity from the point of view of his reputation, but the facts are the facts. I am a great admirer of Orwell, but we have to accept that he did take a McCarthyite position towards the end of his life."

The only people allowed access so far to the notebook containing Orwell's private list of "cryptos" are Michael Shelden, author of Orwell: The Authorised Biography, Bernard Crick, author of George Orwell: A Life, Orwell's late wife, Sonia, and his executors.

Michael Shelden believes that Orwell was assisted in the venture, which was initially for his own private purposes and only made available to IRD later, by Richard Rees, an old literary friend. Mr Shelden quotes Mr Rees as saying that their collection of names was "a sort of game we played - discussing who was a paid agent of what and estimating to what lengths of treachery our favourite betes noires would be prepared to go."

Appearing in the notebook are the heiress and hostess Nancy Cunard, the former Labour politician Tom Driberg, the authors JB Priestley and John Steinbeck, the playwright George Bernard Shaw and the actor Michael Redgrave.

"Beside many of these names are personal comments," writes Mr Shelden. "The scientist Solly Zuckerman is described as being a 'strong sympathiser only', with the added comments, 'Could change. Politically ignorant.' Next to Sean O'Casey's name is the annotation 'very stupid', and next to Kingsley Martin's name is 'decayed liberal. Very dishonest'."

Britain's eminent leftists want to know whether they are also there, perhaps with a scathing comment or a disquieting question mark. The notebook, when eventually available to the public, will read after all like a Marxist honours list. Whether an act of faith in socialism or a betrayal of the left, perhaps Orwell has paid former comrades the most backhanded of compliments.