David Cameron has said he "abhors" the leaking of a civil service memo which said Nicola Sturgeon wanted the Conservatives to stay in power.

The Prime Minister said the leak was "not acceptable" and insisted he wanted to get to the bottom of it.

Ms Sturgeon has categorically denied telling the French ambassador she wanted Mr Cameron to remain in power.

The claim was made in a memo by a Scotland Office civil servant following a talk with the French consul general, who had discussed Ms Sturgeon's meeting with the ambassador, Sylvie Bermann, in February.

The French diplomats have also said the memo is inaccurate.

A probe into the leak has been announced by Sir Jeremy Heywood, the head of the civil service.

Mr Cameron said: "I abhor the leaking of documents. We need to have private diplomatic conversations. I see as Prime Minister the importance of that protocol."

"While there is a proper leak inquiry going on I want to get to the bottom of this.

"This is not acceptable behaviour."

He added: "As far as what Nicola Sturgeon said, I don't know."

Mr Cameron denied the SNP surge - Ms Sturgeon's party is on course to win more than 40 seats according to the polls - was his "secret weapon" in the election.

"My secret weapon is the economy," he said in a briefing for journalists in Edinburgh ahead of a day of campaigning in Scotland, England, Wales and Northern Ireland.

The PM also warned a minority Labour government backed by the SNP was a "chilling prospect".

He said the situation was different from 2007 to 2011, when the Scottish Conservatives supported a minority SNP government.

He said: "I think it is very different in the UK parliament to do a deal with the SNP when they want to break up the country.

"Annabel Goldie (the Scots Tory leader at the time) had a very clear rule. What she did in the Scottish Parliament was right at the time; she had a red line - anything that threatened the integrity of the UK she would not take part in."

In London, George Osborne gave an absolute assurance that the Conservative Party would not resort to dirty tricks and and would play fair in the election campaign.

Asked if, in the light of concerns about the leaking of the false memo, he would make clear the Tories would not use the machinery of government to engage in dirty tricks, the Chancellor said: "I can absolutely give you the assurance we will play by the fair rules established by the Civil Service.

"Indeed, as I understand it - of course, it's not my responsibility - but the Cabinet Secretary is quite rightly conducting an investigation into how the leak, the appearance of that note in the public domain, came about. You can rely on the Cabinet Secretary's impartial approach on that," he added.

Meantime, Mr Osborne, asked at a campaign press conference how the Conservatives could stop the SNP surge in Scotland, replied: "I have a very good answer: vote Conservative."

He insisted that Ruth Davidson, the Scottish Conservative leader, had been the "outstanding leader of the Unionist cause both in the referendum and now in the General Election".

The Chancellor claimed she had "revived" the Scottish Tories, which were now offering not just a clear alternative to the Nationalists but also to the "tired and hollowed-out" Labour Party.

"Ruth Davidson has been a star performer in recent months and a star performer in this election campaign as well. That's the best answer(to the SNP surge)," he added.