ANY pretentions that the UK is a union of equals have been blown apart by Tory belligerence, according to the Westminster official who led the negotiation of the Edinburgh Agreement.

In an explosive critique of Boris Johnson’s response to requests for a Scottish independence referendum, the former Whitehall chief says there is “no good reason” the election of a pro-independence majority in May would not represent a mandate for a second vote.

Ciaran Martin, now a professor of practice in the management of public organisations at Oxford University, lambasted the Tories for their contempt for Scotland during the Brexit process and discredited the “once in a generation” argument against a referendum.

In a column for the Times, he argues claims from No 10 that indyref2 should be ruled out for decades have “shattered” the basic principles underpinning the Union, signalling to Scots that “there is no lawful, democratic path to achieving independence”.

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Martin writes: “To resist a referendum, UK ministers are relying on the Yes campaign’s ‘once in a generation’ soundbite from 2014. But that was just a slogan, with about as much constitutional standing as the £350 million for the NHS on the Brexit bus. It doesn’t bind today’s voters.

“The best measure we have of Scotland’s consent for the Union remains its parliamentary elections. If there is a pro-referendum majority (and this is absolutely not the same thing as an SNP majority), there is no good reason to resist one. Nor is there any reason to alter a set of rules that commanded the confidence of both sides in 2014. Fear of a different result isn’t a reason to ignore Scotland’s election result.”

The former civil servant says David Cameron recognised the right of Holyrood to hold a vote in 2014 but that any consideration for Scotland was abandoned as the Tories turned their attention to Brexit.

The National: The Tories' 'once in a generation” claims have about as much constitutional standing as the £350 million for the NHS on the Brexit busThe Tories' 'once in a generation” claims have about as much constitutional standing as the £350 million for the NHS on the Brexit bus

“Tragically, Westminster lost interest in the Union the moment Scotland narrowly consented to remain in the UK,” he writes.

The professor adds: “Britain’s delicate constitutional balance has subsequently shattered. After 2014 it was briefly fashionable to describe the UK as ‘quasi-federal’. This nonsense was exploded by the Brexit vote which showed that a large majority of Scots could and would be overruled and ignored on a critically important issue.”

With the majority of Scots having rejected Brexit, Martin notes the UK’s negotiating mandate was based on the whims of backbench Tory MPs. “The restraint with which England has historically treated its dominant position within the Union was abandoned,” he writes, describing the post-Brexit constitutional settlement as a “Greater England one”.

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He explains that with the core principle of the Union trampled upon, there is no hope of any federalist solutions being effective.

The former civil servant goes on to call for candour from both sides if there is to be another referendum.

“Independence inevitably means some form of border with England, a different currency, a wait to join the EU, a huge fiscal challenge, and significant administrative disruption,” he argues. “The challenge for nationalists is to convince sceptics that joining the ranks of small, successful northern European states will be worth it.”

But Martin singled out bogus claims from the Better Together side from the referendum, particularly the assertion that an independent Scotland “would be alone and friendless in the world”.

He concludes: “The more pressing question is whether Scotland is allowed to make this choice at all. If you support independence, you have always known that there is a path to it if you convince enough people to vote for it. That is what the UK government proposes to change. London can block a referendum even if Scotland votes for one.

“But that changes the Union we know, based on consent, to one that survives only through force of law. Some democracies, such as Spain, do not allow votes on break-up. Spain does not claim to be a voluntary partnership of willing nations. Britain does. But is it?”

Martin's column comes ahead of the publication of a new paper about the future of the Union, backed by Sir Tom Devine, Scotland’s foremost historian. In it he says that Holyrood is “constitutionally ... nothing more than a large, powerful county council. And during the Brexit process it was treated commensurately”.

The National: Historian Tom DevineHistorian Tom Devine

Martin adds: “If Scots vote next month for a referendum, there should be one,” he says in a report today.

“A vote next month, or at any time after, in favour of any majority – however constituted – of MSPs elected on an explicit pro-referendum mandate, in effect puts Scotland’s consent for the Union on pause.”

The SNP say Martin's analysis destroys the Prime Minister's case against indyref2.

Depute leader Keith Brown said: "This paper blows apart Boris Johnson's strategy for trying to block a referendum and exposes it for the anti-democratic ploy it is.

"Professor Martin's comments are a welcome acknowledgement – from someone at the heart of the UK Government in the negotiations leading to the 2014 referendum – that there is no democratic basis to block an independence referendum if the people of Scotland vote for one.

"The Union must always be based on consent, and to try instead to base it, in Prof Martin's words, on the 'force of law' would be doomed to failure.

"Scotland has a right to decide its own future, and if there is a mandate for a referendum at this election then Boris Johnson has no democratic or moral authority to try and block it.”