The Liberal Democrats have denied being the source of the leaked memo alleging Nicola Sturgeon wanted David Cameron to be prime minister.

Nick Clegg’s party moved to quash suggestions sparked by an interview with the Prime Minister that LibDem Scottish Secretary Alistair Carmichael might have been the source. Cameron said he knew the memo, which came from the Scotland Office, was not leaked by 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 the LibDems accused Cameron of “mudslinging” and stressed the Tories and the SNP were locked in a “Faustian pact” to get the Prime Minister re-elected and boost calls for Scottish independence.

A party 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.”

Carmichael has already confirmed that the memo, containing an account of a private meeting between the First Minister and the French ambassador, was written in the Scotland Office.

Cabinet Secretary Sir Jeremy Heywood has ordered an inquiry into the leak.

Meanwhile, former LibDem leader Sir Menzies Campbell was in Bearsden to meet researchers at the Beatson Institute for Cancer Research to discuss his party’s plans to protect investment in science and research.