Shadow home secretary David Davis said he was writing to Justice Secretary Jack Straw to demand he order a full-scale investigation - saying ministers must have been aware.

An unnamed whistleblower told The Daily Telegraph that the controversial taping of Labour MP Sadiq Khan's conversations with a terror suspect constituent was part of a much wider practice.

The source - said to have "detailed knowledge" of the operation - told the paper they were among "hundreds" of such eavesdropping operations carried out at Woodhill Prison, Milton Keynes.

Murderers and other category-A prisoners - including Soham murderer Ian Huntley - were also said to have been targeted for the recordings now stored at a "top-secret facility protected by armed guards".

Mark Kearney, the former police officer at the heart of the row, claims he was asked to eavesdrop on a second legally-privileged conversation, between a terror suspect and his solicitor.

High-profile lawyers have already spoken out publicly about their suspicions they were being bugged.

The whistleblower told the newspaper: "Mark [Kearney] didn't feel what was going on was right or legal. Every person who came in and saw these terrorist suspects was the subject of an eavesdropping operation. He was put under huge amounts of pressure. Initially, it was just one or two machines but it steadily increased and now covers other category-A prisoners such as murderers."

The newspaper said the policy was introduced after the September 11 terror attacks in the US and initially restricted to a "handful" of prisons but was now said to have been extended nationwide.

Mr Straw has already asked former high court judge Sir Christopher Rose, the Chief Surveillance Commissioner, to conduct an inquiry into the claims surrounding Mr Khan.