Prosecutors say they have all the information they need to decide whether to bring any charges in the "cash-for- honours" case.
Assistant Commissioner John Yates has been heading up the inquiry
Police have spent more than a year investigating whether people were nominated for peerages in return for donations to political parties.
They initially handed their file to the Crown Prosecution Service in April, but were asked to make further inquiries.
The CPS says it is now reviewing the files ahead of a final decision.
The announcement indicates the end of the police investigation, which has seen 136 people interviewed - including Tony Blair and former Conservative leader Michael Howard.
Last week it emerged that Mr Blair had been questioned for a third time by police as a potential witness, shortly before he left Downing Street.
He was the first prime minister to be interviewed by police in the course of a criminal inquiry.
The investigation began after it emerged that secret loans had been made to Labour ahead of the 2005 general election - and that some lenders had gone on to be nominated for peerages.
It was later widened to investigate cover-up allegations and to probe the activities of the other main parties.
Four people have been arrested - including two of Tony Blair's closest aides, Lord Levy and Ruth Turner.
All involved deny any wrongdoing and no one has been charged.
Scotland Yard had been expected to hand over a file on the case in January to the CPS - but it announced, after the arrest of Ms Turner - that a "new development" meant more time was needed.
A police file was handed over in April, but in June it emerged that the CPS had asked police to carry out further inquiries.