The battle to hold a second Scottish independence referendum moves to Britain's top court next week when it hears arguments for allowing a secession vote in a year's time without approval from British Prime Minister Liz Truss and her government.

The UK Supreme Court is being asked to decide whether the semi-autonomous Scottish government, led by the Scottish National Party (SNP) with support from the Scottish Greens who also back independence, can push ahead with plans to hold a referendum without the consent of the British parliament.

At stake is the very future of the United Kingdom. Polls suggest the outcome of any vote on whether to break Scotland's three-century-old union with England would be too close to call.

"The lawfulness or otherwise of the referendum must be established as a matter of fact, not just opinion," Scotland's First Minister Nicola Sturgeon said when she announced plans in June for the vote. "We must establish legal fact."

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In a 2014 plebiscite, which the British government approved, Scots rejected independence by 55-45%.

But, the SNP argues the vote for Britain to leave the European Union two years later was a game changer, as a clear majority in Scotland were opposed to Brexit. One of the main arguments put forward by opponents of a breakaway in 2014 was that an independent Scotland could not join the EU.

Nonetheless, the government in Westminster has held firm that there should be no second vote.

"I'm very clear that in 2014 when there was a referendum, we said it was once in a generation. I'm very clear (there) shouldn't be another referendum before that generation is up," Truss said on Monday.

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"I was partly brought up in Scotland as well as in England. We're a family and we need to stay together."

Sturgeon argues that as voters backed pro-independence parties in elections for the Scottish parliament last year, there was a mandate for them to bring forward a bill to hold a referendum on Oct. 19, 2023.

But, she has recognised any move to independence has to be legal and internationally recognised. She said next year's vote would be consultative, not self-executing, and would require further legislation from both the UK and Scottish parliaments for Scotland to become independent.

Scottish nationalists will be mindful of the fate of Catalan separatists who unilaterally declared independence from Spain in 2017. Spanish courts declared the move illegal, and the bid for secession failed.

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