Police have been asked to investigate claims letters sent in David Cameron’s name led to a breach of election law.
The former Liberal Democrat MP Adrian Sanders has demanded police take action.
He said the general election mailshots last year meant the Conservatives had broken local spending limits.
A Conservative Party spokesman said they did not fall under constituency spending restrictions because they did not name the party’s local candidate.
The letters repeatedly referred to Torbay, telling voters how important it was for them to back the Tories in that constituency.
One concluded: “The only way you can stop Ed Miliband and the SNP taking us back to square one is to vote Conservative here in Torbay.”
The BBC has been shown similar correspondence sent to voters in another seat.
Mr Sanders, who lost his Torbay seat to Conservative Kevin Foster in the 2015 general election, said: “It is a specific targeted mailshot to a voter in a given constituency saying vote for our candidate in that constituency.
“That has to be a local cost, not a national expense.”
A Conservative spokesman said: “Simply referring to the location where the elector lives does not promote any named candidate.
“The literature only promoted the national Conservative Party.
“Such literature was not a local election expense under the RPA [Representation of the People Act] regime, as it was not connected with promoting the election of any candidate.”
Gavin Millar QC from Matrix Chambers said Mr Sanders had a strong case and claims about election literature could lead to by-elections.
He said: “The law says if you’re convicted of the offence of making a false declaration you attract automatically a five-year incapacity for holding public office and that means you must relinquish your seat in Parliament if you’re an elected MP.”
But Mr Millar, a former Labour councillor, added that other national parties also tended to think they could record similar expenses in their national return.
The Conservative agent in Torbay at the general election responsible for reporting local expenses, Alison Hernandez, has since been elected as the local police and crime commissioner.
Devon and Cornwall Police have asked West Mercia Police to investigate earlier allegations of undeclared spending – made after an investigation by Channel 4 News into visits by a Conservative Party campaigning “battle bus”.
The Conservative Party has acknowledged that, owing to an “administrative error”, some accommodation costs for the activists may not have been properly registered.
Ms Hernandez has been referred to the Independent Police Complaints Commission. She denies any wrongdoing.
The law which governs local election spending – the Representation of the People Act – defines election expenses as those incurred “with a view to, or otherwise in connection with, promoting or procuring the candidate’s election at the election”.
The Electoral Commission has previously said such letters are “generally reportable” under national spending limits, but the final position depends on specific “facts and context”.
- In another development, West Midlands Police joined a list of forces looking into allegations of “improper electoral campaign spending returns”. Nine other forces are also investigating whether expenses should have been filed by MPs’ agents as local spending.