BRITISH spies investigated documentary TV series World in Action to root out any communist influence among its staff, a “top secret” file declassified today reveals.

The sinister operation by MI5 has remained under wraps for over half a century and was buried in a Cabinet Office cache of “ultra-sensitive stuff that was too hot to handle at the time,” according to a top historian.

The file shows that, as early as 1962, Whitehall’s official committee on communism conducted a study that found: “No evidence to suggest that Independent Television programmes were unduly slanted towards communism or that the companies were excessively open to communist penetration.”

Although MI5’s vetting of BBC staff is notorious, the spy agency’s surveillance of independent television is less well-known.

By 1969 spooks were particularly concerned about Granada TV’s World In Action, a cutting-edge investigative journalism series.

The Security Service found “no evidence of a conspiracy” at the programme and reported that any interest from the Communist Party of Great Britain had “diminished.”

“Communists are less influential than Trotskyists, who however are too disunited to be able to execute a joint plan,” the file claims.

MI5 was instructed to maintain an “attitude of relaxed vigilance, and to report again in, say, two years’ time.”

“It is right that the Security Service should keep one eye on what is going on in this enormously influential medium,” one official commented.

The 1969 paper was presented to Whitehall’s official committee on subversion at home by Richard “Dick” Thistlewaite.

He was an MI5 officer who had previously served in Palestine, Singapore and Washington, monitoring black US civil rights activist Paul Robeson in the latter.

The Morning Star showed the file to John Pilger, an investigative journalist who worked for World in Action around the time of the MI5 surveillance.

He said the agency’s assessment was “broadly correct” and that there was “no Communist Party influence.”

“It always amused me that the spooks were obsessed with the Communist Party when the real ‘threat’ — if that’s the word — came from dedicated professional journalism that regarded itself as an agent of people, not power.”

Months after the MI5 study World in Action broadcast his film “A Quiet Mutiny,” which exposed how US troops were refusing to fight in Vietnam.

Mr Pilger said it caused “apoplexy” at the time.

MI5 was contacted for comment.