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It would be unsustainable for the UK Government to block a Scottish independence referendum if a majority of Scots want one, according to a leading think tank.

In a new report, the Institute for Government (IFG) also suggests the legitimacy of the union could be undermined if a request to hold another vote is continually rejected.

The document, which focuses on the impact of a no-deal Brexit and the union, was highlighted on the day Prime Minister Boris Johnson announced a deal had been reached with the European Union.

Under a section titled "Mitigating the risks to the union", the IFG wrote: "The Scottish Government intends to hold a second independence referendum.

"This will need the agreement of Westminster. However, if there is a clear majority in Scotland for a second vote on independence, it would be unsustainable and counterproductive for the UK Government to block it indefinitely."

(Image: Getty Images)

At the SNP conference in Aberdeen, First Minister Nicola Sturgeon restated her position that independence must be achieved through a legally binding referendum.

That was despite calls for a "plan B" approach which would involve starting independence talks with the UK in the event that a pro-independence majority is achieved at the next Holyrood election.

On the potential for the UK Government to block another vote, the report read: "Any attempt to proceed with a second referendum without Westminster explicitly devolving the power to do so would almost certainly be subject to legal challenge and probably a boycott from unionist parties and the UK Government.

"A referendum held in this manner would be unlikely to command widespread legitimacy. It's clear that the Scottish Government , and other prominent supporters of independence, do not want to go down the path of an unauthorised referendum."

The report continued: "A continued refusal by the UK Government to devolve the power to hold an independence referendum would become harder to justify if nationalist parties perform well at the next UK general and Scottish parliamentary elections.

"If the UK Government continues to deny a vote in all circumstances, this could further undermine the legitimacy of the union and make for ever-more dysfunctional intergovernmental relations.

"For both practical and principled reasons, the UK Government should ultimately accept that Scotland has the right to secede from the union should a majority of its citizens so desire."

The report also suggested that potential trade and economic barriers could make the case for Scottish independence more difficult in the event of a hard Brexit.

It said: "There is an argument that the case for Scottish independence might in fact become harder for nationalists to make after a hard Brexit.

"The reason is that if the UK leaves the EU single market and customs union, and Scotland leaves the UK in order to rejoin the EU, then this could lead to new barriers to trade and other economic activity between Scotland and England."

(Image: REUTERS)

SNP depute leader Keith Brown said: "It is self-evident that Scotland can, should and must have the choice to determine our own future with independence.

"This report adds grist to that mill, making clear that the decision Scotland makes must be respected.

"It sends a stern warning to Westminster that continuing to refuse a referendum will completely undermine the legitimacy of an already crumbling union.

"And what right does anybody have to deny the people of Scotland that right to choose our own future? It is a completely unsustainable position, already coming apart at the seams.

"Any form of Brexit will hammer Scotland, as we are dragged out of Europe against our will.

"Westminster's assault on democracy is untenable, and Scotland will have a choice."

Pamela Nash, chief executive of Scotland in Union, said: "We voted to remain in the UK in 2014 and we meant it. We were promised the result of that divisive contest would be respected and it would stand for a 'generation'.

"The very last thing that Scotland needs now, amid the Brexit turmoil, is more division, more uncertainty, and more economic harm with a second 'Scexit' referendum.

"When it comes to our financial services industry, leaving the UK would be devastating for the sector, which exports £9 billion to the rest of the UK every year.

"The polls are clear - barely a quarter of people in Scotland support another referendum within the SNP's time-frame.

"It's time for Nicola Sturgeon and Keith Brown to listen to the majority of people in Scotland, not just the nationalist minority."

Scottish Greens MSP Ross Greer said: "We all know that an independence referendum is coming.

"The chaotic Brexit shambles has only increased the appetite for the people of Scotland to have a say in the future of our country, and I'm confident that when presented with that choice they will choose independence in Europe over Boris Johnson's Brexit Britain."