Former Conservative leader Michael Howard has been questioned by Scotland Yard as part of the "cash-for-honours" investigation, he has said.
Mr Howard was interviewed at his London home
He said he agreed to the police request for an interview, and they stressed he was "not under suspicion of having committed any offence".
About a dozen people are believed to have been questioned by police so far, with three arrests made.
All of those detained have denied any wrongdoing in relation to honours.
BBC chief political correspondent James Landale said Mr Howard was the most senior political figure so far questioned.
He said it was widely believed in Westminster to be "likely to be almost certain now" that Prime Minister Tony Blair would also be interviewed.
In a statement on Monday, Mr Howard, MP for Folkestone and Hythe, said: "In response to a request from the police, I agreed to be interviewed by them in connection with their inquiries into nominations for peerages.
"The police emphasised that they wanted to interview me as a witness and that I was not under suspicion of having committed any criminal offence.
"The interview was not conducted under caution. It took place this morning at my home in London."
A Metropolitan Police spokesman said: "A number of people have been interviewed during the course of the inquiry."
The investigation is looking into whether peerages were offered in exchange for cash, after it was revealed that all three main parties were given secret loans ahead of the last election.
Those arrested were Labour's chief fundraiser Lord Levy, the millionaire head of Biotech, Sir Christopher Evans, and head teacher Des Smith, who was on the council of the Specialist Schools and Academies Trust.
They were all bailed earlier this year - most recently Sir Christopher in September.
The probe began after it emerged that four businessmen who gave Labour £4.5m in unpublicised loans were subsequently nominated for peerages.
The scope of the inquiry was extended to cover similar allegations involving the Conservatives.
The party borrowed £16m from 13 wealthy backers.
Mr Howard's former parliamentary private secretary, MP Alistair Burt, said he doubted that Mr Howard would be surprised by the police interest.
"The police have to be careful to be even-handed. It is not a surprise that they would speak to political figures who were in leadership positions at the time," he told BBC News 24.
"It would be more unusual if this was orientated towards one particular political party than any other.
"Hopefully it will make the conclusion of the police that much stronger if they have interviewed everybody that they could."
Liberal Democrat MP Norman Baker said Mr Howard's interview "underlines that the police are taking the matter professionally and seriously."
He said there had been "questions over cash for peerages" since David Lloyd George was prime minister during World War I.
"The only way we're going to avoid this is to take away the power of patronage from the major power leaders and give it to some sort of independent body, or to have an arrangement where the House of Lords is in some way elected."
Scottish National Party MP Pete Wishart said it was possible to raise funds without relying on big sponsors.
"What's wrong with just ordinary good, honest fundraising?
"Certainly what can't happen is a continuation of these events. We have to get Westminster cleaned up and we have to look at sensible solutions about the funding of political parties," he said.