The Crown Prosecution Service has received files from 11 police forces over allegations that the Conservatives broke campaign spending laws at the last election, a spokesman said.
Officers are looking into whether strict spending limits in key seats were breached during the Tory “battle bus” campaign.
The CPS confirmed receipt of files from police forces in:
Avon and Somerset
Devon and Cornwall
Metropolitan Police in London
It is understood Staffordshire Police has also sent a file, but this could not be confirmed by the CPS.
The revelation follows a 10-month investigation by police forces across the country, which could affect several sitting MPs. Election results could even be overturned if prosecutions follow.
Police and the CPS are not naming any MPs or agents who may be under investigation.
The party faces claims that accommodation costs of activists bussed into key constituencies around the country should have been recorded under individual candidates’ limits, rather than as part of the national campaign.
Claims about the Conservatives’ general election spending - as well as that at three parliamentary by-elections - were raised by The Daily Mirror and Channel 4 News. The party blames an “administrative error” for failing to register some accommodation costs.
Media are reporting that police interviewed South Thanet MP Craig Mackinlay last week as they investigate whether campaigners who were drafted in to campaign in marginal constituencies across the country broke the rules.
The Conservative Party has always maintained the Battlebus were a national expense.
But one couple who volunteered told Channel 4 News that their efforts had a local focus, talking to voters about the candidate and issues in the area.
While they campaigned, Gregg and Louise Kinsell were put up at hotels in Taunton, Plymouth and Hayle but none of these hotels appear to have been declared by any of the candidates in those seats, Channel 4 News reported.
Louise Kinsell said: “When you hear that they’re saying that we went down and we were just giving the central government message, no, no we weren’t.
“I’m not going to lie about that. No we weren’t... They’re telling lies about what we did. We duped people on the doors. It feels like cheating and I don’t like that.
“We were on the bus, we know what happened. We know what we were doing, and they know what we were doing.”