Jack Straw, the Justice Secretary, today said that he was "surprised" at the arrest of the Tory frontbencher Damian Green as part of a Home Office leaks inquiry, and added that he understood the concerns expressed by MPs.

He was speaking after it was claimed that detectives accused Damian Green of "grooming" a young civil servant during questioning over the leaking of up to 20 Home Office documents.

Mr Straw defended Jacqui Smith, the Home Secretary, over her refusal to apologise for Mr Green's arrest and the deployment of counter-terrorist officers. He said that in his four years at the Home Office he could not remember a single occasion on which he was informed about the detail of a police investigation, far less invited to comment.

"Everybody who has been made aware of these unfolding events, of course, is surprised by the nature of the raid, and I certainly understand the concern of my parliamentary colleagues," he said.

"If the Home Secretary, any Home Secretary, had offered an apology there would then be a huge furore about the fact the Home Secretary was prejudging the actions and activities of the police without an investigation."

"The Home Secretary is very concerned about this. Of course she understands the significance of a senior Member of Parliament having his offices raided and how unusual this is. It is outside the normal course of events. But anybody who is in the position of Home Secretary, I think, has no option but to take the approach that Jacqui Smith has done."

Mr Green was held and questioned for nine hours on Thursday "on suspicion of conspiring to commit misconduct in a public office and aiding and abetting, counselling or procuring misconduct in a public office".

His homes and offices, including in the House of Commons, were searched and his computer, phone and other communications equipment seized in the raids, which followed the earlier arrest and then suspension of a junior Home Office official.

Ms Smith’s claim not to know that a Tory MP was under investigation was directly challenged by her Shadow, Dominic Grieve. Meanwhile, Michael Martin, the Commons Speaker, was forced to announce that he would make a Commons statement on why the police were allowed to raid Mr Green’s parliamentary office.

In a welter of briefings from police, ministers, civil servants and Commons officials, the most incendiary came from a senior Conservative who said that Mr Green was accused of "grooming his contact" during his detention. "Damian was very angry at this clear attempt to provoke him and did not reply. As a party we want to make it clear that this was grossly inappropriate but symptomatic of the cack-handed way the police have conducted this investigation."

Sources close to the investigation confirmed that they were examining information suggesting that Mr Green encouraged the official, Christopher Galley, 26, to leak documents and may have set him specific tasks.

The Metropolitan Police are investigating claims that Mr Galley, who stood as a Conservative candidate in a council election, sought out Mr Green at a party event. They are also likely to be studying any reply to a letter he sent asking for a job with Mr Green. Senior civil servants said that about 20 leaks were being investigated. The Tories had put the figure at four.

Mr Grieve alleged that Ms Smith "knew very well" that an MP was being drawn into the investigation, but "just decided to sit back on her hands". The Shadow Home Secretary rejected Ms Smith’s claim that it was proper that ministers kept out of police investigations, saying that "all sense of proportion and common sense" had been lost. The police appeared to have been acting on "flimsy and trivial grounds", he said.

The Speaker said that he would make a statement after Harriet Harman, the Leader of the House, questioned the decision to allow police to search Mr Green’s Commons office. Mr Martin faces questions over whether he personally approved the raids and on what basis.

Sir Paul Stephenson, who becomes acting commissioner of the Metropolitan Police today, may withdraw an application to be the next commissioner after a row with Boris Johnson, the London Mayor, over Mr Green’s arrest. One Yard source said: "This is the worst crisis ever — if we call off the inquiry we look stupid and if we go ahead the criticism will be relentless."

Gordon Brown was told that a Home Office mole had been caught before Mr Green was arrested. Treasury officials refuse to say formally whether there is a leak inquiry into the disclosure of information from No 11, including details of the Pre-Budget Report. A source said that the investigation was internal and that the police were not involved as yet.

Other Home Office leaks being investigated are: the complete version of Sir Ronnie Flanagan’s report on the future of policing in February; the information that a disc containing details of 4,000 Dutch offenders that had been sent to Britain had been lost for a year; and news of the loss of data on thousands of prolific offenders.

The leak in August this year that a memory stick containing names of prolific offenders and the names and addresses of 84,000 prisoners had been lost was particularly damaging as it was disclosed just two days after Ms Smith herself was informed.

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