Jacqui Smith attempted to defuse a bitter showdown between MPs and the Commons Speaker today as the Whitehall leaks row escalated.
The Home Secretary announced she would be making a full statement to the House tomorrow on the arrest of Tory frontbencher Damian Green.
The intervention came shortly before Speaker Michael Martin was due to explain why police were not prevented from searching Mr Green's Commons office last week.
MPs from across parties have expressed fury at what they see as a breach of parliamentary privilege, and have been threatening to stage a protest during his statement this afternoon.
Meanwhile, Acting Scotland Yard Commissioner Sir Paul Stephenson strongly defended his force's handling of the investigation, which saw Mr Green arrested and questioned for nine hours in connection with leaks he received from a Home Office "mole".
Speaking at City Hall in London today, Sir Paul said: "The police must be able to act without fear or favour in any investigation, whomsoever may be involved, where there are reasonable grounds to suspect they may have committed criminal offences."
Despite signs that the intensity of the political furore was causing consternation, Sir Paul said the police had known this was going to be a "sensitive" inquiry. He stressed that the force "should be held to account at the appropriate time".
He also confirmed that officers had received written authorisation to carry out the Commons search from Serjeant-at-Arms Jill Pay.
Sir Paul reiterated that politicians had no involvement in the decision to take action against Mr Green.
"I would strongly refute that I or any senior officer under my command have or have allowed any improper influence of our operations or acted for political purposes. That is not what we do," he said.
The Yard announced yesterday that Ian Johnston, chief constable of British Transport Police, had been brought in to carry out an urgent review of the police inquiry.
Also speaking at City Hall, London mayor Boris Johnson admitted speaking to Mr Green after his arrest but said his actions were "entirely right" as the politician was a close friend and colleague.
But the Mayor flatly denied passing any sensitive police information and said he had a "hunch" he would not be charged.
Mr Johnson said he must still be convinced police did not act "disproportionately". He knew the arrest "would go off like a rocket" and cause "huge political convulsions".
Ashford MP Mr Green was arrested last Thursday, and has been bailed until February.
Civil servant Christopher Galley, 26, who is accused of passing him sensitive Home Office documents, has also been arrested and bailed. Both men deny any wrongdoing.
The political exchanges were becoming increasingly angry today, with Business Secretary Lord Mandelson accusing the Tories of using parliamentary privilege as a "smokescreen" to disguise their alleged involvement in criminal offences.
The peer told BBC Radio 4's Today programme that Mr Galley - a former Tory council candidate who had applied for jobs at Conservative Party headquarters - "systematically passed sensitive and classified Home Office papers to the party, apparently in the full knowledge of the Conservative front bench and in complete breach of the Civil Service Code and the law".
"So whilst I recognise that the anger being expressed by some MPs is no doubt sincerely felt by some of them, I also think it is pretty self-serving by Conservative MPs who want to put up a smokescreen to hide their own party's role in allegedly colluding with a Home Office official in breaking the law," he said.
Ms Smith delivered a blistering attack on Tory leader David Cameron and shadow home secretary Dominic Grieve last night, insisting they lacked "respect" for the law and were "unfit" to be in government.
She categorically stated that no one in the Home Office had been aware that an MP was caught up in the police mole-hunt until minutes before Mr Green was arrested.
"Rather than seek to dismiss the offence the police are investigating as 'an antiquated common law misconduct offence', you would do better to show respect for the law and the duty of parliamentarians to uphold the law," she wrote.
She insisted it was vital to prevent leaks from the Home Office, and that principle extended "not just to matters of national security or otherwise covered by the Official Secrets Act".
Britain's top civil servant risked being dragged into the row last night by delivering a thinly-veiled warning to Whitehall officials over leaks.
Cabinet Secretary Sir Gus O'Donnell, who is believed to have authorised police being brought in to track down the Home Office "mole", insisted internal systems were in place for staff to raise "matters of concern".
It was vital that individuals put aside their political beliefs and kept the confidence of ministers, he said.
Sir Gus made no direct reference in his comments to Mr Galley, but his words contrasted sharply with recent claims by Mr Galley's lawyers that he did nothing wrong by handing embarrassing documents to Mr Green.
Fireworks are expected if, as aides suggest, the Speaker tries to restrict questions to short points of order after his statement at 2.30pm.
A number of Tories are considering the "nuclear option" of tabling a motion of no confidence in Mr Martin, which could potentially pave the way for his removal.
Derek Simpson, joint leader of the Unite union, spoke out in support of the Speaker and accused the Tories of a "witch-hunt" based on snobbery.
"Michael Martin is a proud man with strong working-class roots but this has made him a target for the silver-spoon Tories who want a Speaker schooled at Eton rather than a former sheet metal worker. They can't abide a working-class man doing well.
"Michael has been at the receiving end of a string of unjustified attacks from the Tories. This witch-hunt must end. This is not just an attack on Michael but an attack on the working classes. The Tories remain true to form."Reuse content