THE “once in a generation” phrase parroted by Conservatives in their attempt to block a second independence vote is nothing but a slogan and has no constitutional relevance, one of the architects of the Edinburgh Agreement has said.

Professor Ciaran Martin, from the University of Oxford’s Blavatnik School of Government, also warned that any Supreme Court battle between the London and Edinburgh governments would likely be nullified by a change of law pushed through by the Tory majority in the House of Commons.

On the Scottish elections, Martin said that the party political composition of any pro-independence majority is of "zero constitutional significance".

Speaking on the BBC’s Newsnight, the former top civil servant also said that there had been a severe “absence of self-reflection that nine out of 20 Scots had just voted to leave the United Kingdom” following the 2014 vote.

Martin went on: “I think the assumption seems to have been that the issue would go away rather than resurface.

“There was this reliance on a slogan, ‘once in a generation’, but that was just that, a slogan. It’s got the same constitutional standing as the £350 million for the NHS per week on a bus and it doesn’t invalidate the results of the election held yesterday.”

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Martin, who served as constitution director at the Cabinet Office under David Cameron, said the UK was heading towards a “conflict between mandate and law between Edinburgh and London”.

He went on: “That’s a very difficult situation for the UK as a whole. It goes to the heart of what the Union is. Is it a union of consent or a union upheld by the force of law? At the moment we don’t have an answer to that.”

In a report published in April, Martin said that the Union had been one of consent up to at least 2014, when the first independence vote was held.

However, he said that any attempts to block a second vote would fundamentally alter the nature of the Union, making it one based on coercion rather than consent.

The National:

Scottish Conservative Miles Briggs (above), who was also appearing on the BBC show, was asked if Boris Johnson had the “moral authority” to prevent a second independence referendum.

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He said that there had been a vote in 2014 and that had been “once in a lifetime, once in a generation”. He was speaking directly after Martin had said that these phrases have no constitutional relevance.

Speaking later, Martin warned that the Supreme Court battle over the right to hold a second vote would be invalidated by the Tories imposing a law change using their huge majority in the House of Commons.

He explained: “I think it’s a game of constitutional chess where both sides have very different pieces left on the board.

“The pro-independence movement will have a mandate, the Prime Minister and the government of the United Kingdom have the force of law.

“I would expect even if they lost a Supreme Court case the majority is there in the House of Commons to change the law to make sure any referendum cannot take place lawfully.

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“I would dismiss any so-called wildcat or illegal activity because it’s not in the Scottish Government or the Scottish independence movement’s interest to do things like unilateral declarations of independence because that would make the path to an independent Scotland getting its recognition by other countries very very difficult.

“Fundamentally this is a clash of mandate and law. It will be decided by politics and it goes to the absolute heart of the question. We have been told for many decades now that it is a voluntary union, but at the moment there is no lawful democratic path to the pursuit of Scottish independence. That remains the case and that really is the missing part of the constitutional jigsaw that the United Kingdom Government has to answer at some point.”

The constitutional expert also hit out at the Scottish Conservatives who had been speaking on the show saying that they did not wish to see a second independence vote and that there would not be one.

He said they were aiming to “set aside” the major, leading issue of the Scottish elections because of the likely pro-independence majority.