Scotland Yard has said it would not be "appropriate" to hold a police investigation into the expenses of disgraced MP Derek Conway.
Mr Conway apologised to the Commons over the matter
It said a "lack of systems in this case to account for MPs' expenses would severely undermine the viability of any criminal investigation".
The MP was reprimanded by the Commons authorities for paying his student son nearly £40,000 to be a researcher.
He also lost the Conservative whip over the affair.
Mr Conway, who was suspended from Parliament for 10 days and ordered to repay £13,161, has said he will stand down as the MP for Old Bexley and Sidcup at the next general election.
A Metropolitan Police statement said: "The allegations have already been investigated by the Parliamentary Commissioner and then put before the Parliamentary Committee on Standards and Privileges who recommended sanctions that were imposed by the House.
"The CPS advised us today that they are of the view that the lack of systems in this case to account for MPs' expenses would severely undermine the viability of any criminal investigation leading to a prosecution.
"In these circumstances we do not believe that it is appropriate for a police investigation to be instigated."
Commons Speaker Michael Martin is chairing a group looking at reforming the system of MPs' expenses following the Conway affair.
His office declined to comment on the police statement.
But Liberal Democrat leader Nick Clegg said: "This demonstrates that the expenses system is in desperate need of reform.
"The public have the right to expect that their money is being properly accounted for.
"The Speaker's review of expenses must provide the basis for fundamental changes to the system of MPs' allowances."
In January, a Commons standards committee said there was no record of Mr Conway's son Freddie, a student at Newcastle University, doing work at Westminster.
But the MP denied he was a "crook", saying his son had worked from family flat in nearby Victoria, adding: "Lots of MPs have family who work from home."
He said: "He'd fillet post, scrutinise e-mails and stuff envelopes. I am one of many MPs who employ family members. It doesn't mean there's not a job to be done or that they weren't doing it."