SNP leader Nicola Sturgeon has denied telling a French diplomat she would rather see David Cameron as Prime Minister than Ed Miliband.
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The claims come from a leaked memo, published in the Daily Telegraph, of Ms Sturgeon's meeting with the French Ambassador in February.
The memo was written by a British civil servant after a conversation with France's consul general in Edinburgh, Pierre-Alain Coffinier, who was present.
But speaking to Sky News on Saturday, Mr Coffinier said no preference for PM was discussed at the meeting.
So what do we know about this supposed memo - and who will the story damage most?
:: What does the supposed memo say?
"The Ambassador … had a truncated meeting with the FM [Nicola Sturgeon] (FM running late after a busy Thursday …)
"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 (and didn't see Ed Miliband as PM material)".
:: How did it come to light?
The Daily Telegraph claims the allegations are contained in a leaked UK government memorandum.
It was drafted by a Whitehall official after speaking to France's consul general in Edinburgh, Pierre-Alain Coffinier.
He called the Foreign Office, following protocol, to pass on a confidential account of several of the ambassador's meetings in Edinburgh.
Those meetings also included talks with the Scottish Secretary, Alistair Carmichael. The letter appears to have been leaked to the newspaper.
:: What has the reaction been?
Ms Sturgeon was quick to react, angrily denying the claims by tweeting the Daily Telegraph's Scottish Political Editor: "Your story is categorically, 100% untrue … which I'd have told you if you'd asked me at any point today."
Labour Leader Ed Miliband said: "I think these are damning revelations ... in public the SNP are saying they don't want to see a Conservative government, in private they are actually saying they do want a Conservative government.
"It shows at this General Election that if you want the Conservatives out is to vote Labour for a Labour government."
Mr Coffinier has also denied the story, telling Sky News: "It is normal to talk about the political situation in broad terms, but I do not know where this comes from, because it is certainly not in my report that anyone gave any preference."
:: Who do these allegations affect most?
For Mr Miliband it is not great, that one of your potential political partners would view you as "not PM material".
But ultimately it is Ms Sturgeon who could be most damaged. Critics have long believed that the SNP's long-term aim of an independent Scotland is best served by stoking anti-Conservative rhetoric.
With the Tories pledging greater spending cuts and offering what could be a toxic EU referendum - those unpopular measures north of the border, administered by Westminster, could strengthen Scottish nationalism.
But if the strategy was exposed it could turn voters off.
:: Could Labour gain?
If Labour can convince former Scottish Labour voters that by voting for the SNP, Mr Cameron is more likely to be Prime Minister, that could boost Jim Murphy's beleaguered party.
And inversely, increased support in Scotland for Labour, would increase the chance of Mr Miliband walking into Number 10.
:: And who do we believe?
The denials from both Mr Sturgeon and Mr Coffinier are strong. That begs the question of why a civil servant based in Whitehall would record something that wasn't true.
SNP supporters may well believe Ms Sturgeon, while her opponents might seize on it. But the only people who know for certain exactly what was said were the ones present in February's meeting.