Conservative election tactics which saw bus loads of activists turn up to campaign in marginal seats should be probed by police in case they broke strict spending limits, defeated candidates have claimed.
Tory MPs benefited from the efforts of the RoadTrip campaign which involved battle buses arriving in key target seats.
The Conservative Party insisted that spending on the campaign was in accordance with election laws.
A Daily Mirror investigation found the controversial RoadTrip events helped 24 Tory MPs win their local contests but none declared it in their campaign budget.
Labour MP John Mann told the newspaper "it's a matter for the police" while ex-MP Nick Palmer, who was defeated in May 2015 following a RoadTrip visit to his Broxtowe seat said: "It is extremely worrying.
"I think it would be appropriate for the police to review the evidence and there could be calls for by-elections. "
Government minister Anna Soubry held the seat, which was visited by a RoadTrip team and Home Secretary Theresa May in the days leading up to the May 7 election.
Liberal Democrat Paul Burstow was defeated in Sutton and Cheam, where the winning Conservative candidate Paul Scully received a visit from RoadTrip on the eve of the election.
Mr Burstow's local Lib Dem party told the newspaper: "It is important elections are fought on a level playing field and any doubt over expenses occurred taints the process.
"I hope Mr Scully and his agent answer the questions put to them and would like to see a proper investigation."
Another former Lib Dem MP Martin Horwood, who lost to the Conservatives in Cheltenham, told the Daily Mirror: "We knew the Tories ran a huge campaign here. My impression was they broke the spirit rather than the letter of the election spending rules.
"But if there is a hint they might have actually broken the law, then that needs to be fully investigated. I would support calls for an inquiry."
The Conservatives said campaign events organised by the central headquarters, known as CCHQ, to promote the party form part of the national spending return rather than appear in the individual candidate's accounts.
A Conservative spokesman said: "MPs' election expense returns for the 2015 election were completed and returned by election agents in accordance with the law.
"CCHQ campaigned across the country for the return of a Conservative government. Such campaigning would be part of the national return, not local return."