Police officers who arrested Damian Green were wearing secret recording devices usually reserved for terror suspects, it emerged today as Scotland Yard’s investigation into the Conservative MP was further discredited.

The Metropolitan Police have been accused of employing heavy-handed tactics and breaching protocol by subjecting the shadow Home Office minister to a nine-hour interrogation as his home and offices were searched as part of a leak inquiry.

Assistant Commissioner Bob Quick, who headed the inquiry, was already under pressure to withdraw from it after accusing the Tories of seeking to undermine a police investigation. The revelation that the Mr Green was recorded without the MP's knowledge could increase the calls for his removal from office.

The Conservative Party said today that they will not discuss anything related to the Green inquiry because it is an ongoing police investigation.

They have previously given a series of briefings, statements and speeches criticising the police and suggesting that Mr Green was just doing his job by handling information passed to him from the Home Office.

The Met admitted today that a tape recording of Mr Green’s arrest and period with the police before he was taken to a London police station was made “without his knowledge”.

The force said it had now referred the issue to the UK’s covert surveillance watchdog “for their advice”.

“A tape sound recording was made of the MP’s arrest and subsequent period in police charge, without his knowledge, prior to arrival at Belgravia police station from Kent,” the Met said.

“This was authorised at superintendent level to provide an accurate record of anything that may have been said by officers or the MP over a period of nearly two and a half hours.

“This was done with the best of intentions but to ensure total transparency this mater matter has been voluntarily referred to the Office of the Surveillance Commissioners (OSC) for their advice.

“The Crown Prosecution Service are also aware of this information which would be fully disclosable if any proceedings were to take place.”

Mr Green’s arrest and the subsequent search of his offices in the Houses of Parliament provoked a furious exchange between the Conservative Party and Mr Quick, who is the UK’s most senior counter-terrorism officer.

He was forced to make an unreserved apology this week after he accused the Tories of trying to undermine his probe.

The Acting Met Commissioner Sir Paul Stephenson yesterday backed his assistant, saying he believed a “line had been drawn” under the row.

Last week the leaks probe came under fire after an independent review questioned the “proportionality” of the manner of Mr Green’s arrest on November 27.

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