The police decision to question former Tory leader Michael Howard in the "cash-for-honours" inquiry has added to speculation Tony Blair will be quizzed.
Mr Blair has denied any wrongdoing
Downing Street confirmed the prime minister has not been questioned, but many MPs say it is a matter of time.
Mr Howard was interviewed on Monday by police investigating whether honours have been awarded to people in exchange for donations to political parties.
Mr Howard, and all others, have denied any wrongdoing.
The ex-Conservative leader said he had spoken to police as a witness, not a suspect.
Mr Howard said the interview had been held at his London home and was not carried out under caution.
About a dozen people are believed to have been questioned by police so far, three after being arrested.
Mr Howard was interviewed at his London home
Mr Howard is the most senior political figure so far questioned.
The investigation is looking into whether peerages or other honours were offered in exchange for money.
Those arrested and interviewed during the course of the inquiry so far include 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.
Scottish National MP Angus McNeil and Elfyn Llwyd, parliamentary leader of Plaid Cymru, asked Scotland Yard to probe whether the Honours (Prevention of Abuses) Act 1925 had been broken.
The scope of the inquiry was extended to cover similar allegations involving the Conservatives.
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 showed police were "taking the matter professionally and seriously".
Scottish National Party MP Pete Wishart said it was possible to raise funds without relying on big sponsors.
"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.