For the first time in its history three of the four nations of the UK are led by parties that advocate holding referendums on breaking away from the UK. Recently, the first ministers of Scotland, Wales and Northern Ireland signed an agreement to work together on this agenda. Prime Minister Andy Burnham has made devolution one of his guiding principles, although at the Labour party conference he was clear he intended to see off any threat to the integrity of the UK: “Britain is not perfect. It can be better, and we will make it so, but it still stands for something worth saving, worth fighting for.”What chance would a referendum on independence have of passing in each of the three nations? By looking at polling and election results, it’s possible to assess which one is most and least likely to secede. In each case, I’m going to assume that both sides run solid but unspectacular campaigns. ScotlandMy research has found that most Scottish voters have voted along unionist or nationalist lines since 2011. Before then, it was not unusual to see some strategic voting between Labour and the Scottish National Party (SNP), but now voters primarily make their choice within their own unionist or nationalist camp. There aren’t core ethnic, religious or language issues that drive support for Scottish independence. Support is higher in urban, younger, more educated areas and among those who identify as “Scottish-only”, but these aren’t historic divisions. I’ve put lots of demographic and polling data into a calculator to visualise this.In 2014, unionism won the Scottish Independence referendum by 55% to 45%. Now polling shows that a vote would be close. But if Wales left the UK or Reform won the next general election on a platform of policies that are seen as English-focused, support for secession in Scotland would almost certainly rise.Northern IrelandAs far back as 1912, a temporary northern state was discussed with various boundaries proposed before Northern Ireland was established in 1921. Unlike Scotland and Wales, whose boundaries are ancient, Northern Ireland’s borders were deliberately drawn to ensure a comfortable unionist majority. A person’s position on reunification is closely tied to their Irish or British ethnicity, which largely maps with being Catholic or Protestant, although religious participation is low and declining. It remains a deeply contentious debate with active paramilitary groups for and against reunification.Some people have ceased to identify with this binary choice, especially since Brexit. They are represented by two liberal-to-centre-left parties that have no position on reunification: Alliance and the Greens. These parties haven’t hurt nationalism, but have made a dent in unionism’s three conservative parties. In fact, Sinn Féin became the largest party in the Northern Ireland Assembly in 2022 despite not gaining a single seat because the DUP lost three. This emerging group has the casting vote and evidence suggests that over half would vote for reunification, but on today’s numbers nationalism needs about 80% of them to win. Overall, a result of about 55-45% for the union would be expected. Scotland leaving or Reform winning could increase support for reunification, while Donald Trump’s musings probably make no difference.For unification to happen, the Republic of Ireland also needs to say Yes in a concurrent referendum which should pass, but this isn’t by any means certain.WalesPlaid Cymru’s historic breakthrough this year under leader Rhun ap Iorwerth has made it the largest party in Wales. Labour had won everything aside from two European elections in the previous 100 years, but struggled to justify itself after governing Wales for two decades. It was also hamstrung by Keir Starmer’s deeply unpopular Labour government in Westminster.The nationalist party won 35% of the vote for the Senedd (Welsh parliament). Adding in the Greens and minor pro-independence parties puts Welsh secessionism on 43%. That’s remarkable, but a lot of that Plaid Cymru support is soft and could swing straight back to Labour. It’s pretty common for Welsh Labour to do about 10% better at Westminster than at the devolved level. In Wales, constitutional positions are heavily affected by language politics. Welsh speakers in the west are quite likely to endorse independence, while the more English-speaking east is less convinced. Support is particularly low among the 20% of the Welsh population who were born in England and mainly live in eastern and southern areas. Independence struggles to break the 30% mark in polls, but a hefty dose of “undecideds” means unionism has been under 55% for two years. Constitutional questions don’t dominate the conversation in Wales, so there are more people open to changing their minds than in Scotland or Northern Ireland, but this part of the union is probably safe. If a vote were held now, I’d expect over 60% to back the Union. Scotland leaving or a Reform government could change things.In sum, only Scottish independence is feasible in the short term. The others aren’t close, but they are linked to events in the neighbouring nations and in Westminster elections. Scotland leaving could be the breach that gives momentum to reunification and independence in Northern Ireland and Wales. Reform taking over could also drive all three towards the exit. None of this is certain, thought. What’s more, a Reform government would be very unlikely to call referendums in Scotland or Wales even if the numbers were clearly there for independence.Eoghan Kelly does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.