Prosecutors are looking into allegations of electoral fraud surrounding the Conservative Party during the 2015 general election, Sky News understands.
Files from 12 police forces relating to election expenses have been passed to the Crown Prosecution Service.
Sky News understands the files relate to allegations against the Tories.
Police are looking into whether strict spending limits in target seats were breached during the Conservatives' "battle bus" campaign in the 2015 election.
Two dozen Tories are understood to be under investigation over claims they overspent on their local campaigns in which spending limits are tight.
If Conservatives are found to have committed any offence, their political opponents could ask for the contests to be rerun.
A CPS spokesman said it received files from the following police forces: Avon and Somerset, Cumbria, Derbyshire, Devon and Cornwall, Gloucestershire, Greater Manchester, Lincolnshire, Metropolitan, Northamptonshire, Nottinghamshire, West Yorkshire.
A 12th force - Staffordshire - said it had also passed a file to the CPS.
Meanwhile, Will Quince, Tory MP for Colchester, said he had been told by Essex Police he faced no further action after voluntarily attending an interview under caution last January.
And there have been reports fellow Tory MP Craig Mackinlay, who defeated Nigel Farage in Thanet South, was questioned for six hours last week by Kent Police.
Pressure is building on senior Conservatives after leaked emails revealed furious infighting over who is to blame for a spending row.
Those who masterminded the 2015 election campaign are facing questions, after MPs said they felt cast adrift by party officials "covering their own backsides".
The row centres on the Tories' use of the election battle bus to campaign in key seats, and whether spending on hotels and campaign material was incorrectly registered as national spending, which has much higher limits than local spending.
Karl McCartney, MP for Lincoln and one of those under investigation, wrote a bombshell email to the party chairman Patrick McLoughlin attacking the party's handling of the controversy.
In it, he wrote: "We didn't create this mess, the clever dicks at CCHQ (Conservative Campaign Headquarters) did, and I don't see their professional reputations being trashed in the media."
The Electoral Commission is also looking into the allegations which followed a Channel 4 News investigation.