Suspicion last night centred on the police over who gave the authorisation for the alleged bugging of a Muslim MP's conversations with a terror suspect in prison after Jack Straw, the Justice Secretary, said ministers were not involved.

Reports suggested that it was an officer from the Thames Valley force who sanctioned the supposed eavesdropping of conversations in 2005 and 2006 between Sadiq Khan, Labour MP for Tooting in London, and Babar Ahmad, an inmate at Woodhill Prison in Buckinghamshire, whom the US authorities want extradited over claims he ran a pro-terror website.

It was claimed that the officer reputedly involved is facing disciplinary action on an unrelated matter, but will insist that he had the approval of his chief constable in the alleged bugging case.

Last night, Thames Valley Police refused to comment. A spokesman said: "Our instruction is to refer all enquiries from the media to the Metropolitan Police because it is a matter of national security and they are leading on this."

Scotland Yard also declined to comment, saying it did not discuss national security but pointed out that there was an independent investigatory powers tribunal which could consider any complaint about misuse of powers.

Earlier, in an emergency Commons statement, Mr Straw announced that former judge Sir Christopher Rose, the Chief Surveillance Officer - who supervises the overall system whereby police monitor prisoners' communications - would undertake the official inquiry into the alleged bugging with a view to reporting back in a fortnight.

The Secretary of State said the interception of phone calls and other public telecommunications required a warrant, signed by the relevant Secretary of State.

However, the statutory requirements in relation to "intrusive surveillance operations" by the police and other domestic law enforcement agencies were different and subject to a "hierarchy of approvals", depending on the type of surveillance involved, explained Mr Straw.

"In the case of eavesdropping operations, authorisation is required of a chief officer of police or officer of equivalent rank in the Metropolitan Police Service... ministers play no part," he stressed.

Mr Khan has welcomed the inquiry, saying: "I'm obviously keen to find out whether the allegations are true because the implications clearly are quite serious."

Concern at Westminster is high as conversations between an MP and a constituent are regarded as sacrosanct.

A code known as the Wilson Doctrine, espoused by the former PM Harold Wilson in 1966, insists MPs should not have their telephone conversations bugged - a position supported by the UK Government and Opposition.

However, the talks between Mr Khan and his constituent were not intercepted on the telephone but across a table.

In the Commons chamber, David Davis, Shadow Home Secretary, said the alleged bugging had breached "the spirit of the Wilson Doctrine". He told Mr Straw: "By now you should be able to answer the question of who authorised this." The minister insisted it was a matter for the inquiry.

Pete Wishart for the SNP pointed out that MSPs were not covered by the doctrine and argued they should be "given the same protection as MPs". Mr Straw said warrants about police operations in Scotland were a matter for the Scottish Government.

Later, Shami Chakrabarti of Liberty, the civil rights group, welcomed the official inquiry but insisted it was "high time for simpler and stronger surveillance laws with warrants issued by judges, not policemen nor politicians".