The Times has learnt that Robert Edmiston, who converted a £2 million loan into a donation this year to help to avert a potential funding crisis, is one of a number of rich Tory donors who have been questioned by the police in the cash-for-peerages inquiry.

He was questioned by detectives about the secret loan deal that he struck with the Conservative Party which had the effect of circumventing the law that requires all donations of £5,000 or more to be made public.

The revelation that Mr Edmiston, one of the most influential financial backers of the Tory party, has been interviewed by detectives explains why the Conservatives, who gathered in Bournemouth yesterday for David Cameron’s first conference as leader, have been reluctant to intervene in Downing Street’s difficulties.

Only last week The Times disclosed that Ruth Turner, one of Tony Blair’s most senior Downing Street aides, had been questioned. Mr Blair, when he is questioned, will be the first sitting Prime Minister to be be interviewed in a police corruption inquiry.

John McTernan, the Prime Minster’s director of political operations and one of his closest aides, has also been interviewed under caution by Scotland Yard. Mr McTernan, who was interviewed on the same day as Ms Turner, is thought to have been involved in the preparation of names to be nominated for peerages.

In the past weeks the police have also questioned leading Tories, though not under caution, such as Lord Laidlaw, a former Monaco tax exile, whose loans totalled £3.5 million and Johan Eliasch, a Swedish sports equipment tycoon who lent £2.6 million.

Lord Ashcroft, the former party treasurer, who made a £3.5 million loan, was also quizzed. Lord Ashcroft, the founder of the Crimestoppers charity, publicised his loan arrangement unlike the others.

The Conservative Party was obliged to publish the identities of the 13 supporters who made £16 million in secret loans when the police launched the cash-for-peerages inquiry. All 13 have been interviewed by the police. The inquiry began after Mr Edmiston and four Labour businessmen, who had also made secret loans, were blocked by the House of Lords Appointments Commission. The Conservatives, unlike the Labour Party, told the commission in advance about Mr Edmiston’s £2 million loan. His peerage was blocked because of an unrelated tax issue.

A senior Tory party official said: “Unlike the Labour Party we have helped the police. We have nothing to hide.”

The police have arrested Lord Levy, Mr Blair’s per-sonal fundraiser, and Des Smith, who works for the body that seeks sponsorship for the Prime Minister’s flagship city academies.

Sir Christopher Evans, the biochemistry tycoon who lent the Labour Party £1 million before the last general election, is the only donor to have been arrested. All three were bailed.

Police were trying to establish whether there was a conspiracy to give out honours in exchange for loans that could later be converted into gifts.

The questioning of Mr Edmiston came as Cabinet ministers demanded an inquiry into the Midlands Industrial Council, which in three years has given £950,000 to 50 constituency parties including Mr Cameron’s, which received £5,500.

Most of the members of the MIC, which is registered to a terraced house in a Lincolnshire village, are anonymous.

The MIC gives £1 million a year to Constituency Campaigning Services, a support centre for Tory candidates, at Coleshill Manor, a Midlands country house. Mr Edmiston, a key member of MIC, owns Coleshill Manor. He was not available for comment.

Mr Cameron, challenged yesterday on the BBC One Sunday AM programme about the confidential backers of the MIC, said that he knew the identity of them because he had met them. But the party said later that it would continue to withhold their identities. It is taking advantage of a loophole in election law which means that the membership list can remain confidential because MIC is an unincorporated association.

At the weekend the Coleshill Manor centre was described by Conservative Central Office as independent of the party and registered with the Electoral Commission as a “regulated donee”. Mr Cameron contradicted that yesterday when he said Coleshill Manor was “effectively a part of the Conservative Party”.

The commission states that membership associations registered as regulated donees cannot be part of a political party.

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