EDINBURGH The June Brexit referendum 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.
The Scottish National Party (SNP), the biggest party in Scotland's parliament, has said Scotland should hold another independence vote, its second since 2014, if its views on Brexit are rejected.
While most polls show that support for independence has barely moved from the 45 percent who backed it at in the 2014 referendum, a poll published on Wednesday showed that support for independence had risen to 49 percent.
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 nationalist SNP.
LEGALLY, WHO DECIDES?
Britain's national parliament is sovereign and is therefore the only legitimate authority on constitutional issues.
Scotland's government could present a bill in the Scottish parliament saying that a referendum would be called, but such a move would be open to a legal challenge, say constitutional experts.
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 in Edinburgh legally watertight.
This is what happened to allow the 2014 independence referendum.
Many observers believe the May's government would again give its blessing to a secession vote 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.
What may be up for discussion between Edinburgh and London is the timing; the Scottish government may try 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.
COULD SCOTLAND JUST CALL ANOTHER VOTE ON ITS OWN?
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 ont trial this week for staging it.
"If the Scottish Government tried to introduce a referendum bill without prior agreement, then the UK Govt could challenge this in the Supreme Court," said Akash Paun, from the Institute for Government.
"Most people assume the court would rule against the Scottish Government though there is a point of view that simply holding a referendum is not necessarily beyond the competence of Holyrood, since it would just provide a mandate for negotiations with Westminster but wouldn't in itself bring about independence."
Defence Secretary Michael Fallon told the Herald Newspaper last week 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 last month 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?"
"It is becoming ever clearer that this is a choice that Scotland must make," she added.
The issue is likely to be determined, ultimately, by what public appetite for such a vote is perceived to be and will come down to a call by Sturgeon.
Many say she will not call a vote unless she is sure of winning. Others say she has boxed herself into a corner and, arguing that the UK government is ignoring Scotland, is practically obliged to call a new vote regardless.
(Editing by Michael Holden and Guy Faulconbridge)