In an unusual move, the Coalition Government will this morning publish in full its legal advice on the constitutional consequences of Scottish independence from Professor James Crawford, of Cambridge University – regarded, say Whitehall sources, as the pre-eminent authority on international law – and another leading academic, Professor Alan Boyle, of Edinburgh University.
After the Iraq war, this is the only time the UK Government has made its legal advice public. The UK Government paper, which runs to more than 100 pages, comes as Alex Salmond's most senior economic advisers publish their long-awaited economic framework for an independent Scotland.
The First Minister's fiscal commission working group will argue that holding on to the pound immediately after a vote to leave the Union would also benefit the rest of the UK as a key trading partner.
The UK Government's legal opinion contradicts the view highlighted by Nationalists that, following independence, Scotland and the rest of the UK would equally become successor states. It argues the UK would be the "continuing state" and would remain party to all its international agreements while Scotland would be the "new state".
It casts further doubt on the assertion from the SNP Government that, should Scots vote yes in next year's referendum, the process of becoming an independent state could be completed in 17 months by March 2016. Constitutional experts said three years was a more realistic timescale.
Professors Crawford and Boyle reject the notion independence would lead to the creation of two new states and argue it is impossible in law for two states to inherit the same legal personality as the old state. They also reject claims an independent Scotland would revert to its pre-1707 Act of Union status.
Michael Moore, the Scottish Secretary, who will launch the UK Government's paper in Edinburgh today, said it would serve as a "reality check" for voters. "It will reinforce our central message: Scotland gets a great deal by being part of the UK," he said.
A Downing Street source added: "It will say that the overwhelming weight of international precedent suggests that an independent Scotland would become a 'new state' and the remainder of the UK would be considered a 'continuing state'.
"This means that if Scotland became independent, only the remainder of the UK would automatically continue to exercise the same rights, obligations and powers under international law as the UK does.
"The UK is a party to several thousand international treaties – 14,000 treaties are listed on the Foreign and Commonwealth Offices's database."
The legal opinion was commissioned last year by the Foreign Office, the Cabinet Office and the Office of the Advocate General.
Alistair Darling, head of the pro-UK Better Together campaign, said: "This is a formidable legal opinion from two internationally respected lawyers. Their opinions have to be taken very seriously and they can't just be dismissed by the Nationalists."
Holyrood's pro-UK parties welcomed the document but it prompted a furious response from Nationalists. Deputy First Minister Nicola Sturgeon said: "For the UK Government to argue that the UK will be a 'continuing state' and that an independent Scotland would have no rights betrays a near colonial attitude to Scotland's position as a nation and gives lie to any suggestion that they see Scotland as an equal partner in the UK."
She cited academics who believed Scotland and the UK would both be treated as "successor states", but stressed: "The status of Scotland and the rest of the UK following a 'Yes' vote in Autumn 2014 and before Scotland became independent in 2016 will be determined not by assertions of law, but by negotiation and agreement."