The Crown Prosecution Service is to assess evidence of "serious wrongdoing" in the cash-for-honours affair, the BBC has learned.
Assistant Commissioner John Yates has been heading up the inquiry
A 216-page file was handed over by detectives from the Metropolitan Police to the CPS on Friday.
Prosecutors will decide whether anyone should be charged over claims that honours were given in return for money.
Tony Blair and some of his closest aides have been questioned during the 13-month investigation.
The probe, led by Assistant Commissioner of the Met Police, John Yates, had widened in recent months to look into any attempt to pervert the course of justice.
Scotland Yard said that 136 people had been interviewed. All deny wrongdoing.
A spokesman said there had been extensive consultation with the CPS during the inquiry, and this was the 12th police submission - in total they have handed over 6,300 documents.
The police inquiry began after it emerged that secret loans had been made to Labour before the 2005 general election, and that some lenders had subsequently been nominated for peerages.
The probe was widened to include the other main parties.
Four people have been questioned under caution during the course of the inquiry, including Tony Blair's chief fundraiser Lord Levy, Number 10 aide Ruth Turner and Labour party donor, biotech chief Sir Christopher Evans.
The first man arrested, head teacher Des Smith, has been told he will not face any charges. The others remain on police bail.
The BBC's political correspondent Robin Brant said a decision to charge Ms Turner or Lord Levy would be "disastrous" for the prime minister, particularly if it was announced before he leaves office as expected later this year.
He said prosecutors were wary of the "extreme sensitivity" of the case and mindful that the prospect of a trial was "explosive".
But sources in the attorney general's office had warned against expecting any quick developments in the case, saying it was "early days" for the crown lawyers, our correspondent added.
There has only ever been one prosecution involving the law at the centre of the inquiry - the 1925 Honours (Prevention of Abuses) Act - so it could take some time to decide whether to press ahead with a prosecution.
That decision will be made by Carmen Dowd, the head of the CPS's special crime division.
In a statement, the CPS said it had received the file and that the police would be told of the decision "in due course".
It would be made public after the parties concerned had been told, the CPS added.