Could the UK actually split? Experts explain the legal route for pro-independence leaders

As the Scottish National Party gathered in Aberdeen for the its annual conference, UK Prime Minister Andy Burnham had also travelled north of the border. SNP conferences are of course a platform for the party to coalesce around its core aim of Scottish independence. But at Labour’s conference in September, Burnham told colleagues the union is “worth saving” and “worth fighting for” and he was determined not to be “the last prime minister of the United Kingdom”. This was a riposte to SNP leader and Scottish First Minister John Swinney, who had predicted exactly that weeks earlier. Also in September, Swinney and his fellow pro-independence leaders in Wales and Northern Ireland met for a summit where they asserted their right to decide their own futures. At the talks in Cardiff, the leaders signed two agreements – neither of which changes the law. What they do expose, however, is that the three nations stand in very different legal positions – and that two of them have no route forward at all. The party leaders (Swinney of the SNP, Rhun ap Iorwerth of Plaid Cymru, and Sinn Féin’s vice president, Michelle O’Neill, and president, Mary Lou McDonald) signed what’s known as a memorandum of understanding. Read more: Three of the UK’s four nations are led by parties advocating a split. Which is most likely to go? Officially, the meeting was not a summit of first ministers, rather of party leaders. But this didn’t stop unionists in Northern Ireland criticising O'Neill for taking part while serving as first minister of an executive she leads jointly with deputy first minister Emma Little-Pengelly of the Democratic Uunionist Party. Little-Pengelly is her equal in law. Swinney and ap Iorwerth also signed a separate pact between their governments – the Cardiff Agreement. It describes the UK in its current form as “unjust and unsustainable” and commits both devolved governments to press for greater powers over their “constitutional future”. Swinney called it an “important moment”. But it is a political statement – in reality, it doesn’t affect either government’s statutory powers. A mandate for change? The two documents speak about self-determination in different registers. The memorandum is bold: it says the three nations have the right to self-determination and no Westminster government has the right to block democracy. The governments’ agreement is more careful, referring only to the right of citizens to decide their constitutional future “through democratic means”. That difference matters. In international law, the right of peoples to self-determination does not normally give part of a state the right to break away on its own terms, save perhaps in cases of colonial rule or extreme oppression. The UK Supreme Court said as much in its 2022 ruling, drawing on the Canadian Supreme Court’s decision on Quebec. The claim that Westminster has no right to block these nations’ choices is a political argument, not a statement of the law. Three nations, three routes As one of us (Nikos) has argued, only Northern Ireland has a treaty-backed route to exiting the UK. Under the Belfast/Good Friday Agreement and the Northern Ireland Act 1998, the Northern Ireland secretary must call a border poll if it appears likely that a majority would vote for a united Ireland. Even then, the right belongs to “the people of the island of Ireland”. Reunification needs a majority in Northern Ireland and a separate majority in the Republic, a requirement mirrored in the Irish constitution. The two votes need not fall on the same day, but neither counts without the other. Scotland’s route is blocked. The power to hold a referendum has to be transferred through a section 30 order, as was the case before the 2014 independence vote. Successive prime ministers have refused, and Burnham’s response to the Cardiff meeting was a statement saying that he “believes strongly in the union … rather than engaging in constitutional debates or further referendums”. The Scottish government published a draft referendum bill in August, but it will only be introduced once that order is made. While Westminster refuses, the bill is written but unusable. Wales has an equivalent mechanism – section 109 of the Government of Wales Act 2006. It has never been used to lift the reservation protecting the union of the nations of Wales and England. In September, the Senedd (Welsh parliament) backed a motion asserting Wales’s right to hold a referendum. But a motion settles nothing. So in two of the three nations, repeated election victories for independence-supporting parties can be followed by an indefinite refusal to organise a referendum. There is no rule to say when that refusal must end. The Cardiff agreements themselves sit close to a legal line. The Supreme Court has made clear that devolved legislatures cannot use their powers to put pressure on Westminster over the future of the union. Political agreements are not legislation, so they fall outside that test. And signing as party leaders rather than as first ministers adds a further layer of distance. Legislation is another matter. The Scottish parliament’s 2013 referendum bill succeeded only because the power had already been transferred. That transfer came through a section 30 order in 2013, after the two governments, led by David Cameron and Alex Salmond, settled the terms in Edinburgh. Cameron agreed because the SNP had won an outright majority at Holyrood (as the parliament is known), which made refusal hard to sustain. The transfer was time-limited: the order required the poll to be held no later than December 31 2014, after which the competence lapsed. The “once in a generation” referendum returned a vote against independence. Judges at the Supreme Court held unanimously that the 2022 draft, without such a transfer, was outside Holyrood’s competence. The bill was never introduced: the Scottish government’s own senior law officer, the Lord Advocate, put the question to the court first. The problem was not how the bill was drafted but what it was about, and no redrafting could change that. The union between the nations of Scotland and England is reserved to Westminster. The deeper problem is that the UK has no agreed rules for when a sustained democratic demand obliges the UK government to respond. Canada has developed a duty to negotiate, which would be triggered by a clear majority on a clear question. Until the UK finds its own answer, the question of whether the union is voluntary will keep being asked at the ballot box and refused in Whitehall.