The policeman heading the cash-for-honours probe says he expects to pass a file to the CPS in January.
John Yates said the CPS date could change
Scotland Yard's Assistant Commissioner John Yates told MPs the investigation had gained "significant and valuable material" after interviewing 90 people.
He wrote to the public administration select committee, saying "considerable progress continues to be made".
Police have been investigating whether donors received honours in return for cash. All involved deny wrongdoing.
Mr Yates said "the major developments" in the inquiry remained confidential - which he said showed that, contrary to press reports, security surrounding the investigation "remains very tight".
He had reviewed "operational security" after suggestions that the police were leaking information.
Describing the investigation as being in the "final stages", Mr Yates also said the timetable for reporting to the Crown Prosecution Service could change due to factors beyond his control.
"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.
He said his investigative team "have and continue to adopt a thorough, methodical and impartial approach to the investigation.
"This has resulted in the acquisition of significant and valuable material in relation to the development of the inquiry."
He said the inquiry team had interviewed 35 Labour Party members, 29 Conservatives, four Liberal Democrats and 22 non-party members.
In the letter to committee chairman Tony Wright, dated 13 November, Mr Yates apologised for not revealing further details, but said potential criminal proceedings prevented him doing so.
Mr Yates said there was "clearly a great public interest in resolving these matters as soon as possible" and he understood the committee's desire to be given a timetable.
Labour Party: 35
Conservative Party: 29
Liberal Democrats: 4
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."
Committee member Labour MP Paul Flynn said it was "significant" that considerable 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."
Those interviewed are now known to include Labour Party chairman Hazel Blears.
Labour chairwoman Hazel Blears said she had spoken to police
In an interview for GMTV's Sunday programme she said: "The police have been to see me once to discuss the inquiry in general terms and certainly I obviously wasn't around at the time that these transactions took place."
BBC political editor Nick Robinson said there was "no doubt" that Prime Minister Tony Blair would also be interviewed.
The prime minister's office declined to give any official reaction to the letter.
The inquiry was prompted by a complaint from the Scottish National Party.
SNP leader Alex Salmond told BBC News 24 he was pleased that Mr Yates had said progress was being made.
"It's very encouraging for everyone who believes in the rule of the law in this country that there's such a thorough on-going investigation by Assistant Commissioner Yates and his staff.
"For everyone who believes that it's time to muck out the bile of politics and to have faith in the police to do their job - they should be encouraged and heartened by what Assistant Commissioner Yates has had to say today."