The arrest of one of Tony Blair's top aides in the cash for honours row was made after fresh information was uncovered during a search of the Number 10 computer system, according to reports.

The investigation put police at loggerheads with politicians after Ruth Turner was arrested in a dawn swoop on her home.

The News Of The World said it was informed by sources within the Crown Prosecution Service that a "mole" within Downing Street told the police about potentially incriminating emails.

An independent IT expert was then sent in by detectives, with the permission of Downing Street, to look through communications records, it claimed. But the Sunday Telegraph suggested that detectives had obtained high-level permission to "hack" into the IT system remotely.

Senior Labour figures were quick to criticise Scotland Yard's tactics after officers arrived at the home of Ruth Turner at 6.30am on Friday. Culture Secretary Tessa Jowell and former cabinet colleague David Blunkett both said they were "bewildered" and others accused the police of "theatrics".

But Glen Smyth, chairman of the Metropolitan Police Federation, which represents rank-and-file officers, has accused them of trying to "influence the inquiry" and putting them under "undue" pressure. And the Labour chair of the Metropolitan Police Authority, Len Duvall, said no-one should try to "manipulate or pressurise" officers.

Ms Turner, Downing Street director of Government relations, was questioned on suspicion of perverting the course of justice before being released without charge. Ms Turner, who was interviewed by police twice last year, refutes "any allegations of wrongdoing of any nature whatsoever" and insisted she had always offered full co-operation.

As Mr Blair's "gatekeeper" - a post previously held by close allies Anji Hunter and Baroness (Sally) Morgan - Ms Turner holds a key position in Number 10, reporting direct to chief of staff Jonathan Powell and playing a role in controlling access to the PM.

Ms Turner, 36, was appointed as a Downing Street special adviser in May 2005, having previously served on Labour's ruling National Executive Committee. She stood for the party in the 1999 elections to the European Parliament.

She is the fourth person - and the first salaried Government official - to be arrested in the inquiry, after Mr Blair's personal fundraiser Lord Levy, major Labour donor Sir Christopher Evans and headteacher Des Smith.