A Whitehall inquiry is under way into the leak of a memo claiming SNP leader Nicola Sturgeon would prefer the Conservatives to win the election.
Cabinet Secretary Sir Jeremy Heywood ordered the investigation into how the document, published in the Daily Telegraph, got into the public domain.
Ms Sturgeon said the account of her conversation with the French ambassador was "100% untrue".
French officials said she had not expressed a preference for PM.
BBC political correspondent Eleanor Garnier said it was an "extremely serious" issue, with the UK's most senior civil servant investigating the alleged leak of government material in the middle of an election campaign.
"This is not just any old leak," she said, adding that it raised the question of "who would have leaked this and who were they trying to benefit".
In other election news:
- Chancellor George Osborne told the Sunday Telegraph he wanted to double the number of first-time buyers in the next Parliament
- Labour promoted its own plan to encourage banks to fund 125,000 new homes for first time-buyers in England
- Culture Secretary Sajid Javid outlined Conservative plans to protect children from online hard-core pornography
- UKIP dismissed a poll - leaked to the Mail on Sunday - showing party leader Nigel Farage behind the Conservatives in his bid to become an MP and said it had not been "suppressed"
- Mr Farage told the Sunday Times he had spoken to "more than a handful" of Tories about them defecting to his party if the Conservatives form another coalition with the Lib Dems
- The Lib Dems unveiled plans for a £2.5bn care fund to help keep people out of hospitals through improved GP access, a wider range of services in doctors' surgeries and better healthcare in nursing homes
Ms Sturgeon has accused Whitehall of "dirty tricks" over the Daily Telegraph story.
The paper published on its website a transcript of what it says is an official British government memorandum which includes details of a private meeting between Ms Sturgeon and Sylvie Bermann, the French Ambassador to the UK.
Included in a civil servant's summary is the line that Ms Sturgeon would "rather see David Cameron remain as PM (and didn't see Ed Miliband as PM material)".
Writing in the Observer, Ms Sturgeon reiterated her rebuttal of the newspaper's version of events, saying it had been "comprehensively rejected" both by the French ambassador, and the consul general whose account of the conversation is claimed to have been the basis for the leaked memo.
She also appealed to Labour leader Ed Miliband to join her in "locking David Cameron out of Downing Street", even if Labour is not the largest party after the 7 May election.
David Cameron, campaigning in Abingdon on Saturday, said: "What's Nicola Sturgeon told us today? Well she's told us - she's being anointed as this great genius - she's told us something that I said about four years ago, which is Ed Miliband is not up to the job of being prime minister. I think we knew that already."
The Green Party said there were "serious questions" about the leak.
In a letter to Ms Sturgeon, Sir Jeremy said: "You have asked me to investigate issues relating to the apparent leak of a Scotland Office memo that forms the basis of this morning's Daily Telegraph story.
"I can confirm that earlier today I instigated a Cabinet Office-led leak inquiry to establish how extracts from this document may have got into the public domain."