Nicola Sturgeon has unveiled plans to hold a second Scottish independence referendum on 19 October 2023 - nine years after the last one.
It would ask the same question 55% of Scots voted “no” to in 2014: “Should Scotland be an independent country?”
But the SNP First Minister doesn’t have the legal authority to hold that referendum, and Boris Johnson has made clear he doesn’t want to grant it.
So the vote's legality will be determined by the UK Supreme Court in order to settle the argument.
Even if the Supreme Court rules a vote cannot take place, the issue is far from solved and will be a flashpoint in UK politics for years.
Ms Sturgeon has vowed to fight the next election in 2024 on that single issue if she is rebuffed by the courts and the government in London.
So what on earth has actually happened? We walk you through today’s announcement.
What has Nicola Sturgeon announced?
Scotland’s First Minister said she wishes to hold a second independence referendum on Thursday 19 October 2023.
She announced the date in a statement to Holyrood, where the Scottish Government published its independence referendum bill.
The Bill will seek the views of the people of Scotland on whether or not Scotland should be an independent country.
It would ask the same question as in 2014 and take take place in the second half of the current Scottish parliament, which ends in Spring 2026.
Ms Sturgeon wants it at the beginning of that second half, in October 2023, but her wording gives room to hold it later.
The referendum would be “consultative”, meaning the UK government would only later carry out its wishes, but the winning side would always argue it is binding.
Will a referendum be legal?
At the moment, no.
The Scottish Government needs a Section 30 order to legally hold a referendum - but Prime Minister Boris Johnson has made clear this is not going to be granted.
Ms Sturgeon today emphasised she wants an "indisputably lawful" referendum to take place.
So Scotland's most senior law officer, Lord Advocate Dorothy Bain, will refer the Bill to the UK Supreme Court, to see if such a vote would be legal.
Based in Parliament Square, it will rule on whether a referendum can take place if Boris Johnson (or whoever replaces him) refuses to grant permission.
What happens next?
The UK government will respond to the First Minister’s letter to Boris Johnson, making clear that Westminster, not Holyrood, decides on the constitution of the UK.
The Supreme Court will then begin considering whether or not it is legal, which could take some time.
If the Supreme Court rejects the bid to hold a referendum, expect Nicola Sturgeon to issue a furious riposte to Boris Johnson for blocking it.
The First Minister would also focus entirely on having a referendum in the 2024 general election - making the election a de facto referendum in itself, she claims.
She said: “If it does transpire that there is no lawful way for this Parliament to give the people of Scotland the choice of independence in a referendum and if the UK Government continues to deny a Section 30 order, my party will face the UK general election on this single question: should Scotland be an independent country?"
How has Boris Johnson reacted?
Boris Johnson vowed to “study very carefully” Nicola Sturgeon’s bid for a second Scottish independence referendum.
But all signs are he would reject any attempt to hold it.
He said: “We will respond properly but... I think the focus of the country should be on building a stronger economy - that’s what we’re doing.”
A UK Government spokesperson added: “We are clear that now is not the time to be talking about another independence referendum.
“People across Scotland want to see both of their governments working together on the issues that matter to them.
“A decision has been taken by the First Minister to publish a Bill, and the Lord Advocate has made a referral to the UK Supreme Court. UK Government law officers will now consider their response.”
How have Labour and other politicians reacted?
Scottish Conservative leader Douglas Ross has said his party will not participate in an illegal referendum by a “selfish” SNP.
He added: "We won't play Nicola Sturgeon's games. We won't take part in a pretend poll when there is real work to be done.”
Opposition MSPs have told the First Minister to stop putting the "priorities of Scots on the backburner" with the independence "obsession".
Scottish Labour leader Anas Sarwar accused Ms Sturgeon of using her lead post-Covid to "pit Scot against Scot", saying: "Scotland deserves better".
He said: "Isn't it the case that the pandemic Nicola who said she wanted to pull us through is gone.
"And the partisan Nicola Sturgeon who wants to divide our country is back, pursuing a referendum that two thirds of Scots don't want right now."
Scottish Lib Dem leader Alex Cole-Hamilton asked Ms Sturgeon why her "fixation with breaking up the United Kingdom will always trump the needs of the people in the country".
