The arrest of a Tory frontbencher and subsequent police search of his Westminster office provoked yet more angry scenes in the Commons today as Jacqui Smith, the Home Secretary, defended her department's role in the affair.

In a statement to MPs, Ms Smith again insisted that she had no prior knowledge that Damian Green, Shadow Immigration Minister, was to be arrested last Thursday in connection with an inquiry into repeated leaks from the Home Office.

She said that she backed the right of MPs to do their job and hold the Government to account but the “systematic leaking of Government information” raised issues that struck at the heart of Britain’s “system of governance” and "drove a coach and horses" through Civil Service impartiality.

But in an angry reply that goaded the Labour benches into cries of "shame", her Tory shadow, Dominic Grieve, accused Ms Smith of "wilful ignorance" and of "washing her hands" of the responsibilities of her office.

Mr Grieve said the issues at stake were very serious. “They involve basic ministerial oversight over counter-terrorism police operations against a Member of this House. Heavy-handed and incompetent at best, and at worst an unwarranted assault on our democracy," he said.

He added: “We can all agree MPs are not above the law and the police have no place in politics. Nor has this got anything to do with national security. There is not the slightest evidence of this and the Opposition take the integrity of official secrets as seriously as the Government, despite attempts by Government spokesman to smear and spin to the contrary.”

Mr Green was arrested last Thursday and held for nine hours while police searched his home, constituency and Commons offices. Colleagues said that detectives accused him of "grooming" Christopher Galley, a young Home Office official arrested the previous week who has admitted being behind a number of leaks.

Mr Green's treatment angered his parliamentary colleagues and MPs also complained that the police search of his Commons office had breached parliamentary privilege - which Commons officials deny.

In an extraordinary statement to MPs, the Speaker of the House of Commons revealed that the police had not produced a warrant for the search. With his own position under threat, Michael Martin said that he had not personally authorised the raid, which he blamed on the Serjeant-at-Arms, the relatively lowly Commons officer who signed a police consent form.

But Mr Martin's credibility as speaker suffered a 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 his statement to MPs, Mr Martin said that he had been told that the Scotland Yard detectives who carried out the raid had failed to inform Jill Pay, the Serjeant-at-Arms, that she had the right to insist on a warrant before allowing the search.

But Mr Grieve, referring to a letter from the officer in charge of the investigation to the Home Secretary, said that it was "incompatible" with Mr Martin's statement on that point - suggesting that police believe they followed proper procedures in carrying out the search.

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