NICK Clegg has ticked off David Cameron for being "very silly" in playing detective by suggesting the Liberal Democrats were responsible for leaking the false memo that claimed Nicola Sturgeon wanted the Tory leader to continue as Prime Minister.

Mr Cameron made clear he knew the memo, which came from the Scotland Office, was not leaked by Scottish Secretary Alistair Carmichael's Tory deputy.

He said: "I have heard very clearly David Mundell saying it wasn't him; so one does wonder."

It was suggested an aide to Mr Carmichael is to be questioned in the leak inquiry initiated by Cabinet Secretary Sir Jeremy Heywood.

But the LibDems accused the PM of "mudslinging" and stressed the Tories and the SNP were locked in a "Faustian pact" to get him re-elected and boost calls for 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...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."

He added: "The leak was not from a Liberal Democrat and that is the end of the matter."

Later on the campaign trail, Mr Clegg intervened, saying: "It is really very silly - it is election time - for David Cameron to start pointing fingers like that. Alistair Carmichael's been absolutely clear; of course, he didn't leak them."

Noting how it was right that there was a leak inquiry, the DPM added: "I don't know about David Cameron trying to be a sort of one-man detective on all this.

"Let's have the inquiry, look at everything and then, if the person who was responsible has been found, then that person should be held to account."

The leaked memo claimed Ms Sturgeon told Sylvie Bermann, the French ambassador, at a meeting in February that she would prefer to see Mr Cameron remain in power after the election and did not regard Labour leader Ed Miliband as ''prime minister material''.

However, the content was not verified by the newspaper that broke the story which also failed to approach the First Minister or the French for a response.

The FM branded the contents of the memo "100 per cent untrue" and Pierre-Alain Coffinier, the French Consul-General, also denied Ms Sturgeon had said the words attributed to her.