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Scotland Yard detectives have referred four cases of potential fraud by parliamentarians to prosecutors.
Four files of evidence were handed to officials at the Crown Prosecution Service (CPS).
The cases relate to four people, including members of the House of Lords and House of Commons, a spokesman said.
A Metropolitan Police spokesman said: "The Metropolitan Police Service has delivered four main files of evidence relating to parliamentary expenses to the Crown Prosecution Service.
"The files relate to four people, from both the House of Lords and the House of Commons, and will now be subject to CPS consideration on whether there should be any charges. A small number of cases remain under investigation."
Director of Public Prosecutions Keir Starmer must now decide whether there is a realistic chance of convicting the four and what charges, if any, to bring.
The politicians could face charges of fraud or false accounting, with maximum penalties of 10 or seven years.
Scotland Yard launched several inquiries in June into allegations that politicians abused their expenses and allowances. As a torrent of evidence of dubious claims flooded out of Westminster, police focused on individuals who claimed so-called "phantom mortgages".
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Among them is former Labour minister Elliot Morley, who claimed £16,000 interest payments on a property where the loan was already paid off. It emerged he claimed around £800-a-month mortgage interest on his Scunthorpe home for about 18 months after the loan was repaid.
Other politicians believed to be under police investigation include Labour MPs David Chaytor and Jim Devine, peers Baroness Uddin, Lord Hanningfield and Lord Clarke of Hampstead.
