Michael Martin's credibility as Speaker of the House of Commons has suffered another damaging blow when the Cabinet minister in charge of Government business in the House refused to express confidence in him.

Harriet Harman, Leader of the Commons, was asked repeatedly on BBC2's Newsnight last night whether Mr Martin still enjoyed her confidence after the decision to let police search a Tory frontbencher's Commons office

She deflected the question, saying how it was her responsibility to support the Speaker and his officials and to uphold the principles of Parliament, before finally replying: "I am not saying I have got full confidence in anything or anybody; I'm just telling you what the procedures are."

In an extraordinary statement to MPs, Mr Martin yesterday expressed regret for the fact that Scotland Yard detectives had been able to search the offices of Damian Green, the Shadow Immigration Minister, without a warrant, but said that he personally had not authorised the search.

Instead, Mr Martin blamed both the Serjeant-at-Arms, the Commons officer in charge of security who signed a consent form, and the Metropolitan Police for not having made clear to her that she could insist on a search warrant.

Mr Green was arrested last Thursday as part of a criminal inquiry into Home Office leaks and held for nine hours while police searched his home, constituency and Commons offices. Colleagues said that detectives accused him of "grooming" the young Home Office official who has admitted responsibility for a number of politically embarrassing leaks.

Mr Martin used the promise of an independent review and a Commons debate to calm outraged MPs, who saw the police action as a breach of historic parliamentary privilege, but Ms Harman's failure to back him once again - even though she was asked to do so six times - put the spotlight on his position.

As a former civil liberties campaigner, Ms Harman, who is also the Labour deputy leader, has already distanced herself from her Cabinet colleagues over the Green affair. While other ministers preferred to focus on the need to let the police get on with the job, she said last week that "very, very big constituional principles" were at stake.

Meanwhile, the row rumbles on . Boris Johnson, who as Mayor of London, is at loggerheads with Scotland Yard chiefs after declaring yesterday that he did not think the leaks investigation would produce any charges. He also admitted having spoken to Mr Green, even though as head of the Metropolitan Police Authority he has received confidential briefings on the investigation.

This afternoon, the Home Secretary, Jacqui Smith, is to make a statement to MPs on the Home Office's involvement in the row and the issues it raises.

Ms Smith has already insisted she did not order the police probe into alleged leaks of documents to Mr Green and had no prior notice of his arrest. On Tuesday night, she stepped up the war of words in a letter to her Tory shadow, Dominic Grieve, accusing both him and the Conservative leader David Cameron of acting in a manner that showed them to be unfit for high office.

Peter Mandelson, meanwhile, accused the Tories of putting up a "smokescreen" in order to deflect attention from the police investigation.

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