Nicola Sturgeon has announced that a second referendum on Scottish independence will be held on October 19 next year.
The First Minister wrote to outgoing Prime Minister Boris Johnson about her plans to hold a vote following a speech to Holyrood back in June.
Sturgeon told MSPs that if the offer was refused, a consultative ballot would be held instead - but only if the UK Supreme Court agreed Holyrood has the power to do so.
Scottish Tory leader Douglas Ross said he would boycott any "wildcat" vote held by the Scottish Government.
If a referendum goes ahead Nicola Sturgeon will lead the pro-independence side after Alex Salmond spearheaded the 2014 campaign.
The pro-UK side doesn't have a standout candidate after former Chancellor Alistair Darling steered the 'No' side to victory eight years ago.
So, who could lead any potential pro-UK group? Here are four candidates:
Gordon Brown
The former Prime Minister is a strong defender of the Union and set up an organisation to campaign for "positive and radical change in Scotland".
Our Scottish Future produces research and conducts polling on what issues are affecting Scots.
Brown would likely put his head above the parapet, however it's unsure if he would head any group to defend the Union during a referendum.
He made a number of appearances during the 2014 campaign including a passionate speech on the eve of the referendum.
During the speech, he said: "This is not the fear of the unknown. This is now the risks of the known – an economic minefield where problems could implode at any time, an economic trapdoor down which we go, from which we might never escape."
Ruth Davidson
The Conservative peer departed Scottish politics at the last Holyrood elections after she took up a seat in the House of Lords.
Ruth Davidson went head-to-head with Sturgeon on a number of occasions during First Minister's Questions when she was Conservative leader north of the border.
Many Tories would back Davidson to lead any campaign, however she would likely be unpopular with Labour and Lib Dems politicians.
Douglas Ross has called on Davidson on two occasions to help during election campaigns due to her popularity amongst a section of the Tory vote.
Davidson is probably the favourite to take the reins after she announced at an Edinburgh Fringe event that she would return to frontline politics to fight for the Union.
Anas Sarwar
The current Labour leader is on a mission to get his party back to the top of Scottish politics again, therefore a high profile campaign may help his bid.
A recent opinion poll found that Anas Sarwar is now more popular than Nicola Sturgeon among Scottish voters.
His net approval rate was +19 compared to Sturgeon's +12.
The Glasgow MSP faces Sturgeon at FMQs on a weekly basis so knows how the First Minister operates.
Many in the pro-UK camp would push for Sarwar to put his name forward, however it depends if he wants to commit himself to months of independence debate while attempting to hold the government to account on the day-to-day issues such as the cost of living crisis and the NHS.
Alistair Darling
Could the former leader of the Better Together campaign come back to lead a pro-UK group if a referendum takes place next year?
It is unlikely he will re-enter public life to take on Sturgeon but stranger things have happened.
During his victory speech in 2014 Darling urged Scots to work together, he said: "This has not been an easy campaign. Campaigning against Yes for change, it is sometimes more difficult to argue for No.
"We were obliged to point out that some of the arguments for separation were going to cause damage to our country.
“But we had to do that because those risks were real, and it is in my view a tribute to the good sense of the Scottish people that they decided these risks were too great to take.
“So the vote is over. The Scottish people have now given their verdict. We have made a decision for progress and change, for Scotland within the United Kingdom.
“Come on Scotland. Let’s get on with it together.”
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