EDINBURGH (Reuters) - Scottish First Minister Nicola Sturgeon has demanded a new independence referendum be held in late 2018 or early 2019, once the terms of Britain's exit from the European Union have become clearer.
To do so, she needs authorisation from Britain's parliament, which is sovereign.
The Brexit referendum last June called the future of the United Kingdom into question because majorities in England and Wales voted to leave but in Scotland and Northern Ireland most people voted to stay.
Until recently, polls showed that support for independence had barely moved from the 45 percent who backed it in the 2014 referendum. But polls in the last month have shown support for independence rising.
Scotland joined England in a political union in 1707. While Prime Minister Theresa May's centre-right Conservative Party has a majority in the UK parliament, it has only one lawmaker in Scotland and 54 of the 59 Scottish seats are held by the Sturgeon's Scottish National Party (SNP).
Britain's national parliament is sovereign and is therefore the only legitimate authority to legislate on constitutional issues. Such matters are among the "reserved" topics that in general are not devolved to the Scottish Parliament.
With prior agreement between governments in London and Edinburgh, the UK government can devolve power temporarily from Westminster to the Scottish parliament, Holyrood, making a bill to legislate for an independence referendum in Edinburgh legally watertight. This is known as a Section 30 order.
This is what happened to allow the 2014 independence referendum.
Although a spokesman for May criticised Sturgeon for wanting a new vote on Monday, many observers believe May's government ultimately would give its blessing to a secession vote once more if requested, to avoid being seen to stand in the way of the Scottish people as represented in Scotland's parliament.
Both Ruth Davidson, leader of the Scottish Conservatives, and Kezia Dugdale, leader of the Scottish Labour Party, have said the UK government should not block "IndyRef2" as it is known, if backed by the Scottish people. Both parties formally oppose independence and the holding of a second referendum.
A likely point of contention between Edinburgh and London will now be the timing of a new vote; the Scottish government wants to keep the vote within the two year timeframe in which Britain leaves the EU so Scotland has a chance of retaining its EU membership.
The UK government will likely seek to push that timing back.
If the Scottish parliament voted to hold another independence referendum without the consent of Westminster, constitutional experts say the Scottish bill would be legally challenged, ultimately at the UK's highest judicial body, the Supreme Court.
Such a situation would mark a full-blown constitutional crisis.
The Scottish could still choose to press ahead with a symbolic, non-legally binding vote to put moral pressure on the Westminster government, echoing the "consultation of citizens" plebiscite held in the northeastern Spanish region of Catalonia in 2014.
That poll was carried out without legal backing and Catalonia's former regional governor went on trial for staging it. However Scotland's independence referendum legislation, prepared in October in readiness for this potential outcome, tacitly recognises that the UK government will have to sign off on allowing a vote, constitutional experts say.
"If the Scottish Government tried to introduce a referendum bill without prior agreement, then the UK government could challenge this in the Supreme Court," said Akash Paun, from the Institute for Government.
Defence Secretary Michael Fallon last month told the Herald newspaper that Scotland could "forget it" if it expected London to facilitate a new referendum on independence, causing a row with the Scottish government who accused him of arrogance.
Michael Gove, another Conservative lawmaker, said a few days later that the decision on a new vote was down to the British prime minister and that Sturgeon would be "foolish" to try to call one.
After the Supreme Court in London ruled that the devolved assembly in Edinburgh did not need to be consulted on triggering Brexit, Sturgeon asked:
"Is Scotland content for our future to be dictated by an increasingly right-wing Westminster government with just one MP (lawmaker) here," she asked. "Or is it better that we take our future into our own hands?"
Some commentators have argued that Sturgeon had boxed herself into a corner and was practically obliged to call a new vote regardless of public appetite for one.
Reporting by Elisabeth O'Leary