The BBC launched a wartime purge of communists including Ewan MacColl, the folk singer, and his wife Joan Littlewood, the theatre producer, documents declassified by MI5 reveal today.

The secret files show that even with the fight against Nazism at its height, the BBC was pervaded by an atmosphere of anti-communist suspicion and paranoia, reminiscent of the CBS newsroom at the height of McCarthyism a decade later, portrayed in the recent George Clooney film Good Night, and Good Luck. MacColl, born in Salford in 1915, led the revival of British folk music after the war. Littlewood was one of the most important theatre directors of the 20th century and creator of the hit musical Oh What a Lovely War!

Littlewood and MacColl, the father of singer Kirsty MacColl, acted and gave readings for the BBC, but were banned from the airwaves during the Second World War after being identified as 'subversives'. From the late Thirties until the end of the Cold War, MI5 had an officer at the BBC to vet all editorial applicants, stamping the personnel records of anyone suspicious with a distinctively shaped green tag, or 'Christmas tree'. Only a handful of BBC personnel staff knew what the 'Christmas trees' meant. Unsuccessful applicants were never told why they had been turned down and could not challenge the accuracy of the information used against them.

The MI5 files, released today by the National Archives in Kew, south west London, include a 1941 memo by John Coatman, the north regional director of the BBC, expressing fears that MacColl - real name James Miller - and Littlewood could stoke revolutionary fervour among listeners in the north.

'Miss Littlewood, whose real name is Mrs Miller, and her husband, Mr James Miller, are active Communists who have taken a leading part in the organisation of the Communist party and its activities in this area,' it states. The BBC banned Littlewood, whose work included Children's Hour, from the permanent staff, but cleared her for 'ad hoc appearances in musical or dramatic performances'.

MacColl first came to the attention of the authorities in 1932 when the police chief constable of Salford reported him as a Communist Party member of the Ramblers Section of the British Workers' Sports Federation. Police then mounted a 'discreet supervision' of his home in Hyde, Manchester.

He was placed on the Special Observation List when he joined the army in July 1940. He received a favourable report from his commanding officer but deserted two days later.