Speculation is mounting that the prime minister will soon be interviewed by police investigating allegations of cash-for-honours.
Downing St did not comment on the police letter
Scotland Yard's Assistant Commissioner John Yates has described his inquiry as in its "final stages", with a report due to be given to the CPS in January.
However, he has said he is "about to undertake" other inquiries - taken to mean interviewing Tony Blair.
Downing Street has declined to comment on the inquiry.
Ninety people have been interviewed by police. All deny any wrongdoing.
Mr Yates, in a letter to the public administration committee, said his inquiry team had "significant and valuable material" while "considerable progress continues to be made".
He said he hoped to give the Crown Prosecution Service a report in January - depending on the co-operation of the parties and individuals involved.
But he warned that factors beyond his control could alter the timetable.
"It is also subject to any additional lines of investigation that may result from the inquiries I am about to undertake," he said, but did not provide details.
BBC political editor Nick Robinson said that phrase indicated that Mr Blair would certainly be interviewed by police, as "they could no longer justify not interviewing him".
The prime minister's office declined to give any official reaction to the letter.
In a bid to quash suggestions that the police were leaking information, Mr Yates said "the major developments" in the inquiry remained confidential, showing that security around the investigation "remains very tight".
John Yates said the CPS date could change
Police had interviewed 35 Labour Party members, 29 Conservatives, four Liberal Democrats and 22 non-party members.
The letter explained that Mr Yates was aware the committee wanted further details but potential criminal proceedings prevented him from providing them.
Home Office minister Tony McNulty, appearing on Question Time, said he had not seen the letter.
"It would be inappropriate to comment as police minister," he said - but added, "I do think the whole issue does raise for all parties....a whole range of questions."
He added that the attorney general, who potentially has the final say in whether any charges are laid, would fulfil his duty despite being a close friend of Mr Blair's.
"He will exercise the function in the glaring spotlight of publicity....there's no deal to be done behind the stairs."
Committee member Labour MP Paul Flynn said it was "significant" that progress had been made.
"But I believe that most MPs and members of the House of Lords are shocked by what's been going on.
"We have a system in which the second chamber has a great influence on our legislation, and we want to see people there on their personal qualities, on their merit, and not on the amount of money that they have in their wallets."
The inquiry was prompted by a complaint from the Scottish National Party.
Leader Alex Salmond said the progress vindicated the SNP.
"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 enquiry.
"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."
Labour Peer, Lord Mackenzie, former president of the Police Superintendents' Association, said the letter was "fairly telling" but the outcome of the inquiry was still unknown.
"If the police have significant evidence then what we have to assume is that that could mean significant evidence either way.
"It could mean significant evidence that clears people as well as significant evidence that goes towards their guilty.
"I don't know which the the the letter refers to. We'll have to wait and see."