Nicola Sturgeon and Tony Blair, among others, have warned that a vote to leave the EU in June could lead to the end of the United Kingdom.
The theory is that Brexit would be so unpopular with Scottish voters that it would trigger a second independence referendum, and angry Scots would do what they failed to do last time.
Is this doom-mongering, or is there a real danger of the UK falling apart?
How do Scots feel about the EU?
Scotland’s First Minister Nicola Sturgeon has said a “leave” vote in the EU referendum would create an “overwhelming demand” for another #indyref.
Polling certainly suggests that Scots are much more likely to vote to stay in the EU than the English on 23 June.
As we found in another FactCheck, recent research from NatCen Social Research found that around half of English voters wanted to stay in Europe, compared to around two-thirds of Scots and three-quarters of voters in Northern Ireland.
Some politicians have expressed doubts about the real strength of the pro-EU vote, with former SNP leader Gordon Wilson saying he and other nationalists might vote “strategically” for Brexit in the belief that it will hasten Scotland towards independence.
And ex-deputy leader Jim Sillars said he was “astonished” at the number of party members planning to vote to leave the EU. But all this appears to be purely anecdotal.
As things stand, there is a clear pro-EU bias in polls of Scottish voters, and it goes back at least 15 years, according to NatCen Social Research:
What’s this “double majority”?
Mrs Sturgeon has urged David Cameron to agree to a double lock, “meaning that exit from the European Union would only be possible if all four nations agreed to that, something that would ensure that Scotland couldn’t be forced out of the European Union against our will”.
Given the apparent strength of feeling in Scotland and Northern Ireland, this could scupper an exit from the EU even if a clear majority of people living in the UK vote for it.
But the UK government would have to agree to the principle, and it’s anybody’s guess whether they would or not.
It’s possible that we could hold a referendum, get a “leave” vote then have a massive argument about whether or not to go through with an EU pull-out if it was against the wishes of people in Scotland or Northern Ireland.
The new Scotland Bill, currently going through the Lords, strengthens the principle that the Westminster parliament “will not normally legislate with regard to devolved matters without the consent of the Scottish Parliament”.
This opens up the possibility of Scotland challenging the UK government’s right to change all kinds of devolved arrangements that involve the EU without the agreement of Holyrood.
Professor Christine Bell from Edinburgh University told us a vote for Brexit “would trigger a constitutional crisis” within the UK.
“It’s quite hard to say what would happen. There would be a fair amount of chaos.”
Would there be a second indyref?
Possibly, but it’s not inevitable.
The question of who has the legal right to call a referendum was never really settled last time. Instead, the governments of Scotland and the UK reached a compromise – the Edinburgh agreement.
The UK government could refuse to negotiate this time.
If that happened, the Scottish government could try to go ahead and hold another referendum on its own without agreement from Westminster, but this could be challenged in the courts, according to Professor Aileen McHarg from the University of Strathclyde.
“The challenge could come from the UK government, it could come from the Scottish government’s own law officers, or it could come from a member of the public,” she said.
Would Scots vote for independence if there was another referendum?
That is what a lot of people seem to be assuming, but of course we don’t know what would happen.
Polling carried out since the September 2014 referendum actually suggests a small but consistent lead for the “No to independence” side.
Would a vote for Brexit sway Scottish voters one way or the other? Nicola Sturgeon seems to think anger at the result would carry over into a renewed enthusiasm for independence, but it’s possible to see how the opposite might happen.
Prof Bell said Scottish voters would face even more uncertainty and risk after Brexit than they did in 2014.
The question of an independent Scotland’s currency, which emerged as a key issue in the last independence debate, would be even more controversial now. Scotland would probably have to apply to re-join the EU, and it would be expected to adopt the euro.
And what about the border question? If an independent Scotland re-joined the bloc, it would presumably accept the free movement of EU citizens, whereas an England outside the union would be free to draw up new immigration rules.
Would this mean a new “hard” border between England and Scotland for the first time in centuries? It’s hard to predict how Scottish voters would feel about all this uncertainty.
Prof McHarg told us: “The assumption that Brexit would lead to a second referendum and that this would produce a majority for independence – I’m not sure we can rely on it.
“It would lead to a very messy and unpleasant constitutional situation. What the outcome would be is unclear.”