Former Scottish Secretary Alistair Carmichael has accused David Cameron of an "almost psychopathic ruthlessness" that has put the Union in jeopardy.
The Liberal Democrat MP today attacks the Prime Minister for what he describes as a conversion to "English nationalism" to win last month's General Election that is now the biggest threat to Scotland's place within the UK.
Mr Carmichael also hits out at what he describes as "one of the most egregious pieces of self-serving politics ever seen" - when the Conservative leader announced plans to cut Scottish MPs voting rights within hours of the independence referendum result.
The Orkney and Shetland MP is facing calls to resign after he admitted that he authorised the leak of a UK Government memo about First Minister Nicola Sturgeon.
He is currently facing a parliamentary standards probe into the leak of the document, which he claimed he had not seen before being contacted by a journalist.
Mr Carmichael's attack was prompted by a call by the Scottish Conservative leader Ruth Davidson call earlier this week for pro-Union parties to wake up to the threat of independence.
Mr Carmichael writes that "Ruth Davidson is right... the future of the United Kingdom ...is not secure."
Her message needs to be heard in 10 Downing Street, he adds: "It is from there the biggest threat to the Union now comes".
He adds that on the morning of September 19 last year "in one of the most egregious pieces of self-serving politics ever seen (Mr Cameron) spoke to his own backbenchers and party activists".
From then on, he says, "for Cameron, the threat of September 18th had passed and the focus, with an almost psychopathic ruthlessness, switched to the general election on 7th May. Dave the Unionist was dead, long live Dave the English Nationalist."
He accuses Mr Cameron of "surfing a wave of nationalist sentiment in England" to his party's unexpected majority in the House of Commons last month.
The result, he argues, is nationalist governments at Holyrood and at Westminster, putting the Union at risk.
Mr Carmichael predicts that the Union can be saved.
However, he issues a severe warning about its chances, saying that "at the moment almost everything the UK Government does makes that look less than certain".
A Conservative party source rejected the warning last night.
A source said : "Alistair Carmichael's legacy was to turn his party into a toxic force in Scotland.
"He is the last person the Prime Minister should listen to."
Mr Cameron faced accusations he had betrayed Scots voters after he set out plans for 'English votes for English laws' in the immediate aftermath of the referendum result.
The Conservative Government is set to push ahead with the plans, which Scottish politicians have warned would create a "second class" of MPs within the Commons.
During the General Election there was also anger from the SNP and others over a series of attack advertisements the Conservatives ran in England.
These included one notorious example in which the former SNP First Minister Alex Salmond was portrayed as a pickpocket.
In a speech at the Scotland Office in London on Tuesday, Ms Davidson warned: "We who care about the Union, we all need to wake up."
Labour's shadow Scottish secretary Ian Murray called on her to "have a word with David Cameron" whose conduct after the referendum and during the General Election campaign, he said, "did more to risk the future of the UK than anything else".
Labour have accused the Conservatives of cynically talking up the SNP in order to draw votes from the Labour party both north and south of the border.
In the final result the SNP took 56 of Scotland's 59 MPs.
The leaked civil service memo, also released during the General Election, included a second-hand account of a conversation between Ms Sturgeon and Sylvie Bermann, the French ambassador, which said that the SNP leader had indicated she would prefer David Cameron to remain Prime Minister.
Both Ms Sturgeon and Ms Bermann categorically denied the claim.
Mr Carmichael accepted responsibility for the leak and apologised for an "error of judgement" last month.
A Cabinet Office inquiry found that he could and should have stopped the document being released.
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