DAVID Cameron has said he "abhors" the leak of an inaccurate civil service memo which alleged Nicola Sturgeon wanted the Conservatives to stay in power.
The Prime Minister said the leak was "not acceptable" and insisted he wanted to discover who was responsible.
In Scotland as part of a whirlwind campaign tour of the four UK nations, the Prime Minister also defended his party's unofficial alliance with the SNP at Holyrood from 2007 to 2011.
Ms Sturgeon has categorically denied telling the French ambassador she wanted Mr Cameron to remain in power. The newspaper which uncovered the document did not seek a response from the First Minister nor the ambassador or France's consul general before publishing the story.
The claim was made in a memo written by a Scotland Office civil servant.
It recounted a conversation with the French consul general, Pierre-Alain Coffinier, who was present at a meeting between Ms Sturgeon and the French ambassador, Sylvie Bermann, in February.
According the memo, Ms Sturgeon also expressed the view Ed Miliband was not "Prime Minister material".
The French diplomats have dismissed the account, saying Ms Sturgeon did not indicate who she would prefer to see as Prime Minister.
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."
Briefing journalists before a visit to Scottish Widows HQ in Edinburgh, Mr Cameron 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 in return for concessions on police numbers, cash for rundown town centres and other policies.
He said: "I think it's very different in the United Kingdom parliament to do a deal with the SNP.
"They want to break up the country the UK parliament is the sovereign body of.
"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 and she had a red line that anything that threatened the integrity of the UK she wouldn't take part in."
The leaked memo has incensed the SNP, whose supporters believe it is part of a dirty tricks campaign to smear the party.
Andy Burnham, Labour's shadow health secretary, said his party had no need to apologise to the SNP for reacting to the memo before the First Minister and French diplomats had chance to deny it.
He said: "We can only respond to what people are reporting. This is an election campaign.
"It might be the case that the newspaper owes her an apology."
Labour had previously claimed the SNP was in an "unholy alliance" with the Tories in the belief a Conservative victory would boost support for independence.
SNP election chief Angus Robertson said Labour was in "disarray" after Douglas Alexander, Labour's shadow foreign secretary, deleted tweets he sent about the story.
He said: "Labour are paying the price for jumping on the dirty tricks bandwagon - their rush to leap upon this bogus story has backfired badly."
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