An inquiry has been ordered into who leaked a memo to the Daily Telegraph for "transparently political motives" which claimed Nicola Sturgeon privately wanted David Cameron to remain prime minister.
The Telegraph's story claimed the SNP leader told a French ambassador she would prefer to see a Conservative victory at the General Election, prompting the three main parties to attack her as they try to fight off the SNP's surging popularity.
Sturgeon, who says the claim is "categorically, 100%, untrue", had called on Cabinet Secretary Sir Jeremy Heywood to order a Whitehall probe into how the account of her conversation with the French ambassador was obtained by the paper, which he has now done.
She said the story was a sign of "panic" in Westminster over the surge in support for the SNP, and issued a challenge to Ed Miliband to state publicly that he would work with the SNP to "lock out" Cameron from Downing Street in the event of a hung Parliament.
Polls suggest that the SNP is on track to seize dozens of Labour's Sottish seats and Sturgeon's personal popularity is soaring in the wake of a well-received performance in this week's televised leaders' debate.
The latest poll to examine the impact of Thursday's TV debate between seven party leaders showed they all received a popularity boost with Sturgeon, who came away with the biggest advantage.
The Labour leader again ruled out a formal coalition with the SNP, but declined to comment on the possibility of a looser agreement to co-operate if the election result is inconclusive.
"I'm very clear that there won't be a coalition with the SNP. That's not going to happen," he said. "As for how other parties will end up voting on a Labour Queen's Speech, that's a matter for them. I want a majority Labour government."
Miliband did not rule out a post-election deal, short of coalition, under which the SNP might prop up a minority Labour government on a vote-by-vote or "supply and confidence" basis, saying: "What I'm saying very clearly is we are not going to have a coalition with the SNP. As for other post-election possibilities, I'm not getting into that."
The other two main parties attacked the SNP over the Telegraph report.
Scottish Liberal Democrat leader Willie Rennie said: "The First Minister might deny reports of her tete-a-tete with the French but we all know a Conservative-only government is the result of this election that the SNP want to see. A Tory PM governing alone in Downing Street and veering to the right fuels nationalist fires back home.
"Despite her fluffy, positive words about working with the rest of the UK we know what she really thinks. Her sole ambition is to break up the UK."
A Conservative spokesman said: "Ed Miliband still won't rule out a deal with the SNP because he knows he can't get into Number 10 without them - he's in the pocket of Alex Salmond and Nicola Sturgeon has him on a leash.
The note was written by a civil servant following a conversation with consul-general Pierre-Alain Coffinier regarding the meeting between Sturgeon and ambassador Sylvie Bermann, who was on her first visit to Scotland in February.
According to the Telegraph, it said: "Discussion appears to have focused mainly on the political situation, with the FM stating that she wouldn't want a formal coalition with Labour; that the SNP would almost certainly have a large number of seats ... that she'd rather see David Cameron remain as PM." The note went on to say that Sturgeon said Miliband was not "prime minister material".
However the civil servant appeared to doubt whether the report accurately conveyed Sturgeon's comments, adding: "I have to admit that I'm not sure that the FM's tongue would be quite so loose on that kind of thing in a meeting like that, so it might well be a case of something being lost in translation."
Coffinier - who was present at the meeting - confirmed that he had talked "in broad terms" to the UK Government's Scotland Office about the ambassador's visit, but denied saying that Sturgeon had expressed a preference about the election outcome and said he could not recall any casual comment which could have been interpreted in this way.
"I didn't say that," the consul-general told Sky News. "I do not know where this comes from, because it is certainly not in my report that anyone gave any preference."
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Coffinier said Bermann's conversation with Sturgeon was in English. He added: "They discussed the political situation, which is normal, but at no stage did anyone make any comment on their preference regarding the outcome of the election."
A spokesman for Bermann said: "While the Ambassador and First Minister, some time ago, discussed the political situation, Ms Sturgeon did not touch on her personal political preferences with regards to the future prime minister."
Attending an anti-nuclear rally in Glasgow, Sturgeon said: "This story has already been shown to be 100% untrue - having been comprehensively rejected by both the French Ambassador and Consul General.
"The real issue is how a second-hand and inaccurate account of this meeting - which was not even attended by the UK Government - came to be written by a UK Government civil servant and then leaked to Tory-supporting newspapers at the start of a General Election campaign.
"It suggests a Whitehall system out of control - a place where political dirty tricks are manufactured and leaked. And the Foreign Office now appears to be denying the very existence of such a document.
"I am therefore writing to the head of the UK civil service, Sir Jeremy Heywood, requesting an urgent inquiry into the circumstances of such a false account being leaked for transparently political motives."
She added: "Anyone who knows anything about me knows I don't want to see a Conservative government. I'm campaigning to get the Tories out of Downing Street. We've made if very clear that we will lock David Cameron out of Downing Street - the only person who's not made that clear is Ed Miliband.
"We've said that if there are more SNP and Labour MPs than there are Tory MPs, then we will vote to stop a Tory government even getting off the ground. I reissue my challenge to Ed Miliband today to say likewise."