The comments by Geoffrey Robertson QC come as calls were made for an inquiry into claims lawyers' legally-protected conversations with clients in jail are routinely bugged.

Justice Secretary Jack Straw has already launched an inquiry into claims a Labour MP's jail meetings with a terror suspect constituent were bugged at Woodhill Prison, Milton Keynes.

Sadiq Khan's case sparked particular controversy amid suggestions it breached a ban on bugging MPs - and is being examined by Chief Surveillance Commissioner Sir Christopher Rose.

Now the Tories and Liberal Democrats are demanding a fresh probe into claims by an unnamed whistleblower that it is part of a wider practice nationwide.

Mr Robertson said there needed to be an immediate investigation into the claims - warning that if true they could lead to violent offenders being released.

"The end result, if that is the case, is that these cases will have to be brought back to court and in my view the courts will react with such fury as a matter of principle, those whose conversations were bugged will have to be let out," he said.

Murderers and other category-A prisoners - including Soham murderer Ian Huntley - were said to have been targeted as well as terror suspects.

Opposition parties said such a practice would strike "at the heart" of the justice system and suggested it could only have been sanctioned by ministers.

The Ministry of Justice said covert listening operations were "a matter for the police and are undertaken in line with the Regulation of Investigatory Powers Act 2000". But Shadow Home Secretary David Davis said the allegations were so serious they merited a new inquiry.