AN SNP politician has written to police, asking them to fully investigate the Conservative Party into "serious" allegations around its campaign expenditure in the 2015 General Election.

The Tories face multiple police investigations as well as a probe by the Electoral Commission over allegations they breached spending rules ahead of their knife-edge poll win last year.

They centre on whether the party should have listed the costs of bussing activists into key marginal seats under local spending accounts, rather than its national spending budget, which was larger.

Pete Wishart, MP for Perth and North Perthshire, on Sunday asked the Metropolitan police to to examine whether the party attempted to subvert the Representation of the People Act by fraudulently including this candidate expenditure as national expenditure.

Mr Wishart said: "Where many 2015 Conservative candidates are being properly investigated by a number of police forces across England over their campaign expenditure it is now time for the Conservative Party as a whole to be looked at.

"The national Conservative party must address their role in these serious allegations and be properly investigated to see if it systematically subverted the Representation of the People Act.

"There is no doubt that the activists, accommodation, busses and other transport were co-ordinated by the party nationally and this was passed of as national expenditure, even though this activity was specifically targeted to support individual candidates.

"These are very serious allegations and the penalties for those guilty of election fraud could result in large fines and even imprisonment. There are also concerns that the 2015 election was fraudulently won with a co-ordinated breach of electoral legislation.

"I have therefore asked the Metropolitan Police to properly investigate the Conservative Party and if evidence is found prepare a submission to the Director of Public Prosecutions.

"Nothing is more important than the integrity of our democracy and the British public have to be absolutely satisfied that no party won an election by breaking the rules in place to ensure that these elections are fair."

Speaking earlier in the day, David Cameron said he was confident his party can answer allegations it broke electoral laws, but said if there were "misdeclarations or things left out" from its spending accounts then the Conservative Party must put them in place.

The Prime Minister told ITV's Peston on Sunday: "In the end I'm responsible for everything. But I'm very confident that the Conservative Party is gripping this, with the chairman Andrew Feldman (Lord Feldman).

"Lots of political parties have these bus tours, buses that go round different constituencies, and that is a national expense.

"This is all now in conversation with the Electoral Commission and these other investigations so we should let that take its place.

"But I'm confident, the idea of a bus that is a national bus that visits constituencies, I think the Labour Party's done that, the Liberals have done it, we've done it..."

He added: "I don't believe we have done anything wrong. If there were misdeclarations or things left out we have to put those in place but I'm confident we can answer all the questions being put to us."