'Disgruntled ex-police officer blew whistle on bugging of MP'

The allegation came as Justice Secretary Jack Straw announced a full inquiry into how Tooting Labour MP Sadiq Khan was eavesdropped upon as he met a constituent in prison.

Sources believe the officer, who faces disciplinary proceedings over an unrelated matter, blew the whistle on the secret operation.

It was said to have been ordered by a senior figure in the Metropolitan Police without the knowledge of government ministers and probably without Met chief Sir Ian Blair being told.

Despite a furore over the bugging of an MP - with recording devices planted in six prison desks - the Government looked set to clear Scotland Yard of wrongdoing.

A loophole appeared to exist in the so-called Wilson Doctrine, which was widely understood to ban the bugging of MPs, that allowed intrusive surveillance to be carried out by the police but not, paradoxically, by MI5, the security service.

That came as a shock to campaigners who were under the impression that since 1966 both police and security chiefs had been barred from listening in on MPs.

There was speculation that Mr Straw would announce a root and branch reform to give greater protection to MPs' private conversations with constituents in future.

Earlier, the Prime Minister's spokesman drew reporters' attention to the exact words used by former premier Harold Wilson in 1966 when he spelled out the rules governing buggings and MPs.

Mr Wilson only referred to a ban on the bugging of phone conversations - whereas Mr Khan was bugged by a device placed in a table at Woodhill Prison, where he was meeting constituent and childhood friend Babar Ahmad, who is fighting extradition to the US on accusations of running a pro-terror website.

No10 refused to say how the 1966 statement should be applied in practice, but the implication was that Scotland Yard would only have been in the wrong if it had bugged Mr Khan's personal telephone.

A written Commons answer last September by Gordon Brown stated the Wilson Doctrine also applied to any intercepts that have to be authorised by a Cabinet minister. In the Khan case, the bug would have been authorised by the Surveillance Commissioner, so it was exempt from the ban.

Whitehall sources said MI5 was not involved in the operation, which left the finger pointing clearly at Scotland Yard's anti-terrorism branch. There were suggestions that the Khan bugging did not reach Sir Ian's desk but was handled at assistant commissioner level. Mr Straw and the Prime Minister appear to have known nothing.

Intercept evidence could be used in courts for the first time under legal changes due to be announced by ministers this week. The reform follows a review of the law on phone-tap evidence by a group of Privy Councillors led by Sir John Chilcott. It is understood to have concluded that the evidence should be available to prosecutors in a small number of terrorist cases.