Neither David Cameron nor David Mundell, the new Scottish Secretary, knew about the leaking of the inaccurate Sturgeon/Bermann memo beforehand, a senior UK Government source has made clear.

Earlier this week, Downing Street, when asked if the Prime Minister was aware of the memo before publication, referred to the findings of an inquiry into the leak by Cabinet Secretary, Sir Jeremy Heywood.

Sir Jeremy's investigation concluded that those who "had access" to the memo had been interviewed and the only people "who had any involvement" in the leaking were Alistair Carmichael, the former Scottish Secretary, and now the only Scottish Liberal Democrat MP, and his then special adviser, Euan Roddin.

But this still left open the question as to who, if anyone, was made aware of the memo before it was leaked.

Asked if either Mr Cameron or Mr Mundell knew beforehand of the leak, a well-placed Whitehall insider told The Herald: "I can categorically say they did not. No blue knew."

The source claimed that the leaking of the memo was a purely Lib Dem exercise, which took place during the General Election campaign. "It was badly handled," he said.

The insider added that the PM, who made political capital out of the revelations contained in the memo on the campaign trail, was very angry when he learned they had come about because of an official leak from the Scotland Office. The leak was published in the Conservative-supporting Daily Telegraph.

The memo was a written record, made by a UK Government civil servant, of a conversation in March between Nicola Sturgeon and Sylvie Bermann, the French Ambassador to the UK.

It claimed that the First Minister had told Mme Bermann that she wanted Mr Cameron to remain as Prime Minister and thought Ed Miliband was not up to the job of replacing him in No 10.

This was the direct opposite of what Ms Sturgeon was saying in public; that she wanted to "lock out" the Tory leader from Downing Street and help put the Labour leader in. The FM categorically denied the claims in the memo, saying they were "100 per cent untrue"; the French Embassy also denied them.

The Heywood inquiry found the unnamed civil servant believed his note of the conversation between Ms Sturgeon and Mme Bermann was accurate but accepted the key part, relating to any mention of Mr Cameron and Mr Miliband, "might well have got lost in translation".

It is not known how long Mr Carmichael had sight of the memo before he decided to leak it. The leaking happened within hours of Ms Sturgeon's impressive performance in the first TV leaders' debate and appears to have been timed to cause her maximum political damage.

Earlier this week, Alex Salmond, the former First Minister, referred to Mr Mundell and asked: "Are we expected to believe that he knew nothing whatsoever of the now notorious attempt to smear Nicola Sturgeon and the decision of his then boss to leak to the Tory house journal the Daily Telegraph?

"In a department with so little to do he must have seen the infamous memo. And yet when it was mysteriously leaked he said nothing at all. Mundell is about to be asked the age-old question: 'What did you know and when did you know it?'

According to the senior insider, the new Scottish Secretary knew nothing about the leaking of the Sturgeon/Bermann memo.