By Andrew MacAskill
LONDON (Reuters) - Scottish First Minister Nicola Sturgeon will on Thursday demand the right to hold a new independence referendum, challenging Prime Minister Boris Johnson to keep the United Kingdom together as he grapples with its coming split from the European Union.
A month before Britain embarks on its momentous journey out of the EU, Sturgeon will say she has won a mandate to call for a fresh independence vote after winning 47 of the 59 seats in Scotland in last week's election, 11 more than in 2017.
Sturgeon, who heads Edinburgh’s pro-independence devolved government, will publish a document called "Scotland’s Right to Choose", demanding Britain's parliament transfer the power to Scotland's devolved parliament to authorise a new referendum, setting the stage for a constitutional stand-off.
It is ultimately up to the British parliament to decide whether Scotland can hold a new referendum, and Johnson’s government has repeatedly said it will reject any demand for another vote.
"There is a clear mandate for this nation to have the power to decide its own future," Sturgeon will say, according to extracts released by her office. "The result of last week’s general election makes that mandate unarguable."
Scots rejected independence by 55 to 45 percent in a 2014 referendum but a three-year political crisis in London and differences over Brexit have strained the bonds that tie the United Kingdom together.
Every region of Scotland voted to stay in the EU in 2016, while the United Kingdom as a whole voted to leave, leading Sturgeon to claim a new referendum is justified as Scotland is now being dragged out of the bloc against its will.
If Scotland voted for independence it would mean the United Kingdom would lose about a third of its landmass, almost a tenth of its population, a core ingredient of its identity, and rip apart the world's fifth biggest economy.
Opinion polls suggest Scottish support for leaving the United Kingdom is far stronger than it was before the start of the 2014 campaign, although most polls suggest there is not currently a majority in favour.
In her speech, Sturgeon will reiterate she will only seek to secede from the United Kingdom through a properly agreed referendum.
"We understand that a referendum must be accepted as legitimate, here in Scotland and the UK, as well as in the EU and the wider international community," she will say.
Unionists say independence would needlessly break up the United Kingdom and usher in years of financial, economic and political uncertainty.