Nick Clegg’s party moved to quash suggestions, sparked by an interview with the Prime Minister, that Lib Dem Scottish Secretary Alistair Carmichael might have been the source.
Mr Cameron said he knew the memo, which came from the Scotland Office, was not leaked by Mr Carmichael’s Tory deputy David Mundell. He said: “I have heard very clearly David Mundell saying it wasn’t him, so one does wonder.”
But a Lib Dem spokesman said: “David Cameron’s attempt at mudslinging does a terrible job at masking what’s at the heart of this matter. The Scottish people have a right to know that the Conservatives and SNP are involved in an invisible Faustian pact.”
“He wants Sturgeon to triumph in Scotland to pave his way back to No 10, while the SNP secretly yearn for a Tory win as they misguidedly believe it will increase their chances of another Scottish referendum.”
“The leak was not from a Liberal Democrat and that is the end of the matter.”
Mr Carmichael has already confirmed that the memo, containing an account of a private meeting between First Minister Ms Sturgeon and the French ambassador, was written in the Scotland Office.
Cabinet Secretary Sir Jeremy Heywood has ordered an inquiry into the leak of the note, which claims that Ms Sturgeon told ambassador Sylvie Bermann that she would prefer to see Conservatives remain in power after the May 7 poll.
The note went on to say that Ms Sturgeon said she did not think Labour leader Ed Miliband was ‘’prime minister material’’.
Ms Sturgeon has claimed the contents of the memo are “100% untrue”.
It was written by a British civil servant following a conversation with consul-general Pierre-Alain Coffinier regarding a meeting between Ms Sturgeon and Ms Bermann, who was on her first visit to Scotland in February.
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