The policeman investigating the loans for peerages row has said he has uncovered "significant and valuable material".
In a letter to MPs, Metropolitan police assistant commissioner John Yates says he expects to pass a file to the Crown Prosecution Service (CPS) in January.
However he notes this is "subject to any additional lines of investigation that may result from the inquiries I am about to undertake" - raising speculation he is about to question Tony Blair.
Downing Street refused to comment on the letter and has so far stressed that it has had no formal contact with the Scotland Yard officers investigating the honours affair.
In his letter to the public administration committee, dated November 13th, Mr Yates says 90 people have been interviewed so far.
This includes 35 members of the Labour party, 29 Conservatives, four Liberal Democrats and 22 others. All deny any wrongdoing.
He says that "considerable progress continues to be made" in the inquiry, adding: "This has resulted in the acquisition of significant and valuable material in relation to the development of the inquiry."
Police are investigating claims that parties offered wealthy businessman a seat in the House of Lords in return for a financial contribution, and that this money was made in loans rather than donations to keep them out of the public eye.
All three main parties reject the allegations and have promised to cooperate fully with the inquiry, which is looking at possible breaches of a 1925 act prohibiting the sale of honours, and a 2000 act requiring the declaration of party donations.
Paul Flynn, a member of the public administration committee, told BBC News 24 that Mr Yates' comments were "clearly very serious" and expressed his hope that the probe reaches a "speedy conclusion".
"I believe most MPs and members of the House of Lords are shocked by what's being going on," he said.
"The second chamber has great influence on our legislation and we want to see people there on their qualities, on their merit and not on the amount of money they have in their wallets."
Alex Salmond, leader of the Scottish National Party (SNP) who made the original police complaint about the handling of honours, said the letter proved the probe was serious.
"I think everybody in this country who wants to see politics cleaned up, who wants to see a situation where people do not buy their way into the legislature, will be extremely encouraged at the Metropolitan police conducting such a thorough-going inquiry," he said.
"Remember almost six months ago, when my young colleague Angus McNeil raised this matter, people were laughing - all the old lags at Westminster said 'oh no, the police'll never stand up to Downing Street'. Well, they're not laughing now."
To read John Yates letter visit the public administration committee website.