The leaders of Scotland, Wales and Northern Ireland's devolved governments gathered in Cardiff on Tuesday to sign a joint memorandum of understanding calling on Westminster to "prepare, plan and facilitate constitutional change" in their respective nations — marking the first time all three of the UK's devolved territories have simultaneously been governed by nationalist or pro-independence parties.

Plaid Cymru leader Rhun ap Iorwerth, Scottish National Party leader John Swinney, and Sinn Féin's Michelle O'Neill — who serves as First Minister of Northern Ireland in a power-sharing arrangement — were joined by Sinn Féin's all-Ireland leader Mary Lou McDonald. The document they signed asserts that no Westminster government has the right to "block democracy" by refusing a referendum should the public demand one. Swinney and O'Neill called for immediate referendums in Scotland and Northern Ireland respectively, while ap Iorwerth declined to set a timetable for Wales, where polling currently puts support for independence at around 32%. Scotland and Wales aspire to full independence and potential re-entry into the European Union; Sinn Féin's goal for Northern Ireland is reunification with the Republic of Ireland. The three leaders were careful to describe the Cardiff meeting as a gathering of party leaders rather than heads of government, a distinction that carries weight: Northern Ireland's unionist Deputy First Minister Emma Little-Pengelly of the Democratic Unionist Party sharply criticised O'Neill, arguing she had "no mandate to represent Northern Ireland at the summit or to sign binding texts."

The meeting takes place against a backdrop of heightened constitutional debate across the UK. Prime Minister Andy Burnham has made devolution a central pillar of his agenda, but has repeatedly closed the door on referendums — last week telling the Commons that a Scottish independence vote was "off-limits" while his government focuses on the economy and cost of living. His position on Northern Ireland was similarly firm, with London insisting it will adhere to the 1998 Good Friday Agreement, which ended three decades of conflict and stipulates that a border poll can only be triggered when a clear majority appears to favour a united Ireland. The gathering was also lent additional context by Donald Trump's recent remarks that he would "love" to see a united Ireland, comments that drew swift rebukes from unionists and the UK government alike.

Why this matters: for the first time since devolution was established in the late 1990s, the political arithmetic across the UK's "constituent nations" — the term for Scotland, Wales and Northern Ireland as distinct components of the United Kingdom — is aligned in favour of those who want to leave it. Even if Westminster shows no immediate sign of relenting, the Cardiff agreement signals a new phase of coordinated pressure. In Scotland, independence support hovers near 50%; in Wales it is growing, bolstered by Plaid Cymru's historic defeat of Labour in the Senedd elections in May. The three parties have also pledged closer cooperation on economic policy, energy, and relations with Europe regardless of the constitutional outcome — suggesting that, even short of independence, a more assertive Celtic political bloc is taking shape.