The police officer leading the investigation into the cash-for-honours scandal today turned up the pressure on Downing Street claiming that "significant and valuable material" has now been uncovered.

John Yates, Scotland Yard's Acting Assistant Commissioner, said the discoveries had been made during 90 interviews that his team had conducted as part of the investigation so far.

The disclosure that his team have embarked on significant inquiries, made in a letter to MPs today, will further heighten speculation that Tony Blair could be interviewed imminently.

The letter, dated November 13, was written in response to a request for more information from a cross-party committee on November 7.

Mr Yates rejected claims by Downing Street that his team have been leaking details of the inquiry to the media, insisting it would not be in their interests. He also implied that there have been major advances in the probe that have not emerged publicly.

"I am confident that [operational security] remains very tight. This is endorsed by the fact that the major developments in this inquiry are not in the public domain," he said.

Three people have been arrested so far as part of the investigation: Lord Levy, Mr Blair's personal fundraiser, Des Smith, a Government adviser, and Sir Christopher Evans, the biotech tycoon. All insist they have done nothing wrong.

Two of the Prime Minister's top advisers, Ruth Turner, Downing Street's director of government relations, and John McTernan, have been questioned under caution. The inquiry has seen 90 interviews conducted so far - 35 from members of the Labour Party, 29 from Conservatives, four from Liberal Democrats, and 22 from non-party members.

Ms Turner, who became the director of Government relations at Number 10 in May last year, is believed to have been asked about e-mails and documents relating to the inquiry. Mr McTernan is thought to have been involved in the nomination process for peerages.

All of those concerned with the inquiry have denied any wrongdoing and nobody has yet been charged.

Police have also questioned Ian McCartney, the former Labour Party chair, and Lord Sainsbury, the former-science minister who resigned last week, but not under caution.

The former Health Secretary Alan Milburn, a close ally of Mr Blair and one of the architects of the 2005 General Election campaign, announced last week that he had been questioned as a witness.

Patricia Hewitt, Health Secretary, has confirmed that police have asked to interview her, but a spokeswoman said today that a date had not been set.

But the Prime Minister, who has admitted being aware of the loans, has not yet been asked by detectives for a written statement, leading to speculation that they could be waiting until the closing stages of the inquiry to approach him. His official spokesman said today that Mr Blair had not been contacted by police.

Michael Howard, the former Conservative Party leader, was also interviewed last month.

The police investigation is considering whether party officials or donors have breached the Honours (Prevention of Abuses) Act 1925, which makes it an offence to take money as an "inducement or reward" for procuring an honour for someone else.

They are also considering the Political Parties, Elections and Referendum Act 2000, which restricts the sources of donations to political parties and makes it an offence to evade those restrictions.

Mr Yates met with Hazel Blears, the chair of the Labour Party, on October 5 to update her on the progress of the investigation. A spokesman for Ms Blears said: "It was a courtesy meeting to keep her up to date. This was the first time they have met to discuss the matter." Ms Blears is not being treated as a suspect or a witness in the inquiry, the spokesman insisted.

However, Tory sources said they had not received any updates from detectives on the inquiry.

Last week it was disclosed that virtually all of those serving in the Cabinet in the run-up to last year's General Election had been asked to provide statements "formally in writing" of what they knew about secret loans to the Labour Party.

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