Lord Sainsbury of Turville, the Science Minister, and Ian McCartney, the Trade Minister, were questioned by detectives from Scotland Yard. Neither minister was cautioned.

A spokesman for Mr McCartney said: “He voluntarily offered to speak to police. He was interviewed as a witness, but he wasn’t cautioned and did not attend a police station.”

When Mr McCartney was Labour Party chairman, he signed the nomination papers for peerages for four businessmen who secretly lent the party more than £5 million. He signed them from his hospital bed, where he was recovering from surgery.

Mr McCartney, who was demoted to Trade Minister in this year’s Cabinet reshuffle, told the police that he had not been aware of the loans made by businessmen whose peerages the House of Lords Appointments Commission blocked.

Lord Sainsbury, who was questioned by John Yates, the Deputy Assistant Commissioner, who arrested Lord Levy on Wednesday, lent the Labour Party £2 million last year but failed to declare the loan. The minister, who was made a life peer in 1997 and has given £6.5 million to Labour since 2001, had to apologise in April when details of the loan were revealed.

However, the Prime Minister has cleared him of any breach of the ministerial code.

Police also questioned several senior Tories, including Lord Marland, the Conservative Party treasurer, as part of the inquiry into whether peerages were sold for money. The crossparty Constitutional Affairs Select Committee that is examining party political funding wanted to talk to Alastair Campbell, the Prime Minister’s former spokesman, about the amount of pressure exerted on Lord Levy to deliver millions to fight the last general election.

However, Mr Campbell, who left Downing Street in 2003 and returned before the last election, said that his role in the campaign was not central enough to warrant giving evidence. Select committees have the power to compel witnesses to attend but Mr Campbell’s explanation was accepted.

Lynton Crosby, the Australian media guru who directed Michael Howard’s election campaign, has agreed to give evidence to the committee next week.

The Public Administration Committee, which is running a separate inquiry into whether peerages were sold for cash, has delayed its own inquiry after private briefings from Scotland Yard.

Two interim reports by police have been sent to the Crown Prosecution Service. A decision on whether charges should be made is expected in the autumn, close to the party conference season.

The police are also preparing to question other Downing Street officials, including the chief of staff, Jonathan Powell, the political director, Ruth Turner, who is also the link between Mr Blair and the trade unions, and Richard Roscoe, the head of the honours unit.

So far police have questioned 48 people, 13 of them under caution, and made two arrests — Lord Levy, Mr Blair’s personal fundraiser, and Des Smith, who works for the body that seeks sponsorship for the City Academies. Both men were bailed.

Mr Blair’s official spokesman said that Metropolitan Police had still not made an official request for an interview with the Prime Minister. An interview, however, is expected to take place next month when Mr Blair returns from holiday.

The spokesman declined to comment on whether any Downing Street officials had been approached for an interview or whether those questioned would be able to draw on the advice of government lawyers. “We don’t comment on police investigations, full stop,” he said. “That’s the right and proper approach of the Government.”

He said that it was a matter for ministers to confirm whether they had been questioned. “It has always been clear that Downing Street will co-operate fully, but that doesn’t mean that I am giving a running commentary on a police investigation,” he said.

Lord Sainsbury was one of the first prominent businessmen to declare for new Labour when Mr Blair became party leader in 1994. He is Britain’s 29th richest man and his £1.6 billion fortune is derived from the supermarket empire that bears his name.

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