The Scotland Yard investigation centres on allegations that Labour members may have breached honours legislation by putting wealthy donors forward for seats at the House of Lords.

A key aide of prime minister Tony Blair has been questioned for a second time by police investigating the cash-for-honours affair.

According to the London Evening Standard, the prime minister's director of government relations, Ruth Turner, was quizzed at a London police station yesterday.

Ms Turner was first questioned last month after being arrested on suspicion of breaching the Honours (Prevention of Abuses) Act 1925 and on suspicion of perverting the course of justice.

A spokeswoman for the Metropolitan police confirmed that a woman returned as per her bail conditions yesterday.

'She has been released on bail pending further enquiries,' the spokeswoman said, adding that no date had been set for her to return for further questioning.

Downing Street said it would not be commenting on the reports, 'as per usual with this investigation'.

Lord Levy, Labour's chief fundraiser; Des Smith, a government advisor; and Sir Christopher Evans, who loaned the Labour party £1 million in the run-up to the last general election, have all been arrested as part of the investigation.