He said the bugging had produced numerous tip-offs, including a warning that led to the deployment of tanks at Heathrow airport in 2003 because of a suspected terror plot.

He added: “It was widely known, but not officially acknowledged, that bugging took place. The information was never used as evidence – it would have been totally inadmissible – so I do not see how you could reopen a case like Huntley’s on that basis. It was only ever used as intelligence.”

Huntley was jailed for life in 2003 for the murders of 10-year-old friends Holly and Jessica in the Cambridge-shire town the previous year.

South-east Cambridgeshire Tory MP Jim Paice last night said he was astonished the authorities would have done anything that could have derailed his conviction.

He said: “If there is any risk people like him could go free as a result of this it would be disastrous. If this bugging really is so widespread it suggests a section of our security services has got completely out of control.”

Shadow Home Secretary David Davis said it was  “inconceivable that this action has taken place without ministerial approval”, and promised he would be writing to Justice Secretary Jack Straw to demand an inquiry into the claims.

And Lib Dem home affairs spokes-man Chris Huhne described the allegations as “appalling”, and warned bugging breached the fundamental legal right of defendants and their lawyers to private conversations.

Reports yesterday said an unnamed whistleblower claimed “hundreds of conversations” had been bugged at Woodhill through microphones hidden inside specially adapted tables.

Former Detective Segeant Mark Kearney has claimed he was asked to listen in on Mr Khan’s conversations during his time at Woodhill. He faces a misconduct trial over allegations that he leaked stories to the media.

Last night a Ministry of Justice spokesman said: “Police monitoring operations are a matter for police and are undertaken in line with the Regulation of Investigatory Powers Act 2000.

“The Prison Service may grant the police permission to operate in prison providing there are no concerns about order or control. Such co-operation is vital in the fight against serious crime and terrorism.

“It would not usually be appropriate for the Prison Service to question a police decision to monitor an individual, nor would it expect police to share details of intelligence cases underpinning its monitoring operations.”