Scotland Yard has blamed the chaotic state of Commons expenses for scotching criminal proceedings against disgraced MP Derek Conway.
The Met said prosecutors had advised them that the "lack of systems" in Westminster would "severely undermine the viability" of any police probe.
The Old Bexley and Sidcup MP was severely reprimanded by the Commons authorities in January following allegations that he paid two sons tens of thousands of pounds from the public purse despite having done little apparent work.
Mr Conway has since been stripped of the Tory party Whip and announced he will be standing down at the next general election - although he will still draw his £60,000 salary until then.
The scandal sparked renewed outrage over the level of public funds available to Parliamentarians in expenses, and the lack of transparency and accountability.
Last month Met Commissioner Sir Ian Blair suggested that he backed a formal probe into Mr Conway's actions, insisting that police would not "ignore the situation".
However, in a statement, the Yard pointed out that Westminster sleaze watchdogs had already looked into the situation.
"The CPS advised us 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," the statement said. "In these circumstances we do not believe that it is appropriate for a police investigation to be instigated."
Mr Conway was suspended from the Commons for 10 days and ordered to repay more than £13,000 that was improperly handed to his student son for acting as a researcher.
The Commons Standards and Privileges Committee said it could find no evidence that Freddie Conway had done any work whatsoever in the job, despite receiving more than £45,000 from staffing allowances over four years.
