A dozen police forces around the country have passed files to the Crown Prosecution Service as part of an investigation into Conservative election expenses in 2015, it has emerged.
The forces are understood to have conducted interviews and investigations into possible breaches of election law relating to how campaign funding was declared by the party.
Police have focused on the Tory Battle Bus tour, which visited a number of different areas during the campaign and was staffed mainly by volunteers.
Over 20 Conservative MPs could be involved and the CPS will decide whether the cases should be heard in court by the end of May.
It came as a second Conservative MP admitted he was interviewed under caution over expenses concerns but Essex police said they would take no further action.
Will Quince, the MP for Colchester, said in a statement that the complaint against him had been "vexatious and politically motivated".
He is the second Conservative MP to be questioned after the Telegraph revealed Craig Mackinlay, the South Thanet MP, was subjected to questioning for over six hours on Friday.
Mr Mackinlay won the South Thanet seat in 2015, beating Nigel Farage to enter the Commons.
Senior Conservatives aides and advisors reportedly traveled to the seat to campaign and the party threw significant resources behind the fight in a bid to cause embarrassment for Ukip.
The MP's office declined to comment, but a source close to Mr Mackinlay confirmed he was interviewed under caution as part of the investigation on Friday last week. A Tory source said: "They really put him through the mill, he was there for six hours."
A Conservative spokesman said: “We are cooperating with the ongoing investigations.”
One of Theresa May's key aides Nick Timothy was reportedly involved in the campaign, while Channel 4 News revealed last night that another two senior members of staff also traveled to the constituency during the campaign.
Stephen Parkinson, Theresa May’s Political Secretary and Chris Brannigan, Director of Government Relations at the Cabinet Office were both named in secret emails about the campaign in South Thanet.
Speaking about his police interview Mr Quince added: "Politics is not a game. I would ask those individuals to think about the cost of this investigation, the important work those police officers could have instead been doing over this lengthy period, the stress that it put me, my family and my team under and the reputational damage to me personally."
The CPS yesterday confirmed that all files are "under consideration". The forces which have submitted details include: Avon and Somerset, Cumbria, Derbyshire, Devon & Cornwall, Gloucestershire, Greater Manchester, Lincolnshire, Metropolitan, Northamptonshire, Nottinghamshire, Staffordshire and West Yorkshire.
Other MPs have directed their anger at Conservative Campaign Headquarters (CCHQ), complaining that they had been cut adrift by the party's high command even though the complaints relate to the busing of activists to campaign in key marginal seats, which was organised centrally.
The allegations centre on whether the "battle bus" tour should have been recorded as counting towards individual candidates' spending limits, rather than as part of the larger national spending return.
Earlier this week in a leaked email to Conservative Party chairman Sir Patrick McLoughlin, Lincoln MP Karl McCartney complained they felt "completely cast adrift" and had been "left to fend for themselves".
In a statement, Mr McCartney said he had made clear his "forthright views" privately to a number of senior party figures on behalf of backbenchers.