The documents, which include details of surveillance between the 1920s and 60s, indicate not only the wide range of groups and individuals being watched by police but also officers' spectacular ability to misjudge what they saw. The obtuseness of some exasperated their superiors.

A Sergeant Ewing of Special Branch, monitoring Orwell's attempt to recruit Indians to work for the BBC's India service in January 1942, noted: "This man has advanced communist views ... He dresses in a bohemian fashion both at his office and in his leisure hours."

A Home Office official named W Ogilvie - whose pencil was probably responsible for a question mark against Ewing's statement - responded a few days later: "I spoke to Inspector Gill of Special Branch asking whether his sergeant could elaborate on the question of Blair's 'advanced communist views'. Mr Gill rang me up this morning to say that Sergeant Ewing described Blair as being 'an unorthodox communist' apparently holding many of the views but by no means subscribing fully to the party's policy.

"I gathered that the good sergeant was rather at a loss as to how he could describe this rather individual line, hence the expression ... This fits in with the picture we have of Blair@Orwell [sic]. It is evident from his recent writings ... that he does not hold with the Communist party, nor they with him."

The authorities finally decided Orwell was not a communist from his answers to a questionnaire posed to leading leftwing figures and published in Left magazine, including the question: "Should Socialists support the British war effort?" to which he answered, "yes".

By then, Special Branch had been monitoring Orwell for more than 10 years.

Orwell, who was born Eric Arthur Blair in India in 1903, worked for the Burma police in the 1920s before heading to France, where the then penniless writer first came to the attention of MI6.

The file on "Orewell", as his name is spelt at least once, starts with him as Mr EA Blair in January 1929, when he was offering to work as Paris correspondent of the Workers' Life, the forerunner of the Daily Worker. Within a month Scotland Yard noted he was also claiming to be Paris correspondent for the Daily Herald and Daily Express.

It continues in the 30s as he helped out at a leftwing bookshop, Booklovers' Corner in Hampstead, where he was a friend of the owner, Francis Westrope: "[He] and Blair are on friendly terms and the latter is known to spend a good deal of time at the shop. He has on occasion conducted the business. Westrope is known to hold socialist views and considers himself an 'intellectual'."

The file contains a cutting from the Manchester Guardian in September 1938, noting that Orwell had signed the Joint Peace Manifesto alongside the Peace Pledge Union, the Quakers and the Labour party. Two years earlier, the chief constable of Wigan had requested information about the writer, who had been seen addressing Communist party meetings in the town.

There are also details from his passport application, noting: "height 6ft 2ins, eyes grey, hair brown, tattoo marks on the backs of both hands".

The police never found enough on the author of Animal Farm and 1984 to prevent him obtaining a passport or being accredited as a war correspondent for the Observer. A record in the file, dated 1942, describes him as someone who "has been a bit of an anarchist in his day and in touch with extremist elements". It adds that he had "undoubtedly strong leftwing views, but he is a long way from orthodox Communism".

Among others kept under watch in the 1950s was the American folk song collector Alan Lomax, who appeared regularly on the BBC.

Special Branch monitored the programmes, noting his association with the Scottish singer Ewan McColl "who is connected with Theatre Workshop, a dramatic company with communist connections", and even the film director Joseph Losey (left), labelled a communist sympathiser. A concert in St Pancras town hall in December 1953 was deemed suspicious because its theme, Songs of the Iron Road, with "melodies originating on the railway system" was advertised in the Daily Worker.

The BBC was asked to keep an eye on Lomax, but he was allowed to continue broadcasting and his folk song programme preceded the Queen's Christmas Day message in 1957.

A file from 1941 contains 13 pages of reports on a strike by dockyard apprentices in Chatham which followed a complaint about the standard of canteen food.

It includes a report by Sergeant Ivan Smith of the Kent police, who hid himself in the gents' toilet at a working men's club in Gillingham to listen to the speakers at a union meeting: "I was unable to hear the majority of the proceedings owing to my obscure position in the lavatory and to the fact that a boy was standing in the cloakroom door the whole time," it says. "I heard someone say that the best thing would be to have all the big fellows for that job [on the picket line]. The reply was that all present at the meeting should take their share and no shirking. The meeting closed at 1630 hours but I was unable to leave until 1655 as some of the apprentices were in the hall until that time."

The interrogation files of suspected spies and German agents have also been released, including, in that of a Norwegian seaman, a copy of the Naturist magazine of March 1945 whose photographs of nude women and advertisements for breast enhancement and "the Vitaman iodised jockstrap" were combed to see whether they contained writing in invisible ink.

One bulky file contains interviews over years with a Russian defector, Leon Helfland, who had been a KGB officer and assassin, and charge d'affaires in the Soviet embassy in Rome for seven years until he fled to the US in 1940. Helfland told an astonished British consular official named WH Gallienne over lunch in New York in 1941 that the Soviets, Italians and Germans had read every telegram and document from the British embassy in Rome from 1933: "Mr H said they often marvelled at our laxity."

Helfland never named the spy involved, but it is known to have been the Italian valet of the British ambassador, Sir Eric Drummond, who refused to believe that his servant could have betrayed Britain.



· This article was amended on Wednesday September 12 2007. George Orwell was born in India in 1903, not 1902 as we said in the above article. This has been corrected.