The investigation into cash-for-honours has turned up "significant and valuable material", according to the detective leading the probe.
In an letter updating MPs, Acting Assistant Commissioner John Yates also said that "considerable progress continues to be made".
He said his Scotland Yard team had conducted 90 interviews so far - 35 from the Labour Party, 29 from Conservatives, four from Liberal Democrats, and 22 people not linked to any particular party.
Mr Yates told the House of Commons Public Administration Select Committee he hoped to be able to send a file of evidence to the CPS in January - as long as individuals and parties cooperated.
The letter, dated November 13, was written in response to a missive from the cross-party committee on November 7 requesting more information.
Mr Yates rejected claims that his team have been leaking details of the inquiry to the media, insisting it would not be in their interests.
He also implied that there have been major advances in the probe that have not emerged publicly.
"I am confident that (operational security) remains very tight. This is endorsed by the fact that the major developments in this inquiry are not in the public domain."
In the letter, Mr Yates said his team was entering the "final stages" of its investigation.
"It is my view that considerable progress continues to be made. The 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."
