Leaders of Scotland, Wales and Northern Ireland are set to meet in Cardiff on Monday for a landmark summit that will call for the three nations to have the right to hold independence referendums.

John Swinney, Scotland’s First Minister and leader of the Scottish National Party (SNP), Welsh First Minister Rhun ap Iorwerth of Plaid Cymru, and Northern Ireland First Minister Michelle O’Neill of Sinn Féin are expected to sign a memorandum of understanding setting out closer cooperation between their administrations.

The agreement is expected to cover cooperation on the economy, energy, relations with Europe and the principle of national self-determination. The meeting represents an increasingly coordinated effort by the pro-independence parties leading the devolved governments in Edinburgh, Cardiff and Belfast.

The Cardiff summit comes after major political changes in all three nations. Following the 2026 elections, Scotland, Wales and Northern Ireland are all led by parties that support greater national self-government or independence. Plaid Cymru’s Rhun ap Iorwerth became Wales’s first minister in May, marking the first time the party has led the Welsh Government since devolution.

Swinney: current UK system is “not sustainable”

Mr Swinney has intensified his campaign for a legally recognised route to another Scottish independence referendum in recent weeks.

He has argued that Scotland should have a mechanism comparable to Northern Ireland’s arrangements under the Good Friday Agreement, which provides a legal framework for a referendum on Irish unification when the relevant conditions are met.

Earlier this month, Swinney wrote to UK Prime Minister Andy Burnham urging him to turn his recent comments about majority public support into an agreed legal mechanism for a Scottish referendum. Swinney said the people of Scotland, rather than Westminster, should ultimately determine the country’s constitutional future.

His intervention followed comments by Burnham suggesting that a referendum question should be considered if there were a clear change in public opinion. Downing Street has subsequently sought to clarify that the Government’s position on Scottish independence has not changed and that there is currently no commitment to hold another referendum.

Wales seeks greater constitutional powers

For Wales, the meeting comes as Plaid Cymru begins its first term leading the devolved government.

Ap Iorwerth has previously called for greater powers for the Senedd, including responsibility for areas such as policing and justice, while arguing for a stronger and fairer relationship between Wales and Westminster. He has also stressed the importance of cooperation with Scotland on issues where the two governments share interests.

The House of Commons Library notes that Plaid Cymru’s 2026 election manifesto committed the party to pursuing the powers needed to hold an independence referendum and to devolving decisions over the timing, question and process of such a vote to Wales.

Northern Ireland adds a different constitutional dimension

Northern Ireland’s participation gives the initiative an additional constitutional significance because its future status is already subject to a distinct legal framework.

Under the Northern Ireland Act 1998, implementing the Good Friday Agreement, a border poll on Irish unity can be held when it appears likely that a majority would support a united Ireland. Swinney has explicitly pointed to this mechanism as a potential model for Scotland.

The three governments are therefore not simply seeking greater cooperation on devolved policy. Their discussions are also likely to highlight their shared argument that constitutional decisions affecting Scotland, Wales and Northern Ireland should ultimately be determined by voters in those nations.

Growing pressure on Westminster

The Cardiff meeting comes at a particularly sensitive moment for the future of the Union.

The UK Government has faced renewed pressure from the SNP over a second Scottish independence referendum, while Plaid Cymru has begun pushing the question of Welsh self-determination higher up the political agenda. At the same time, the constitutional question in Northern Ireland remains linked to the possibility of Irish reunification.

The Institute for Government has warned that the new political landscape requires Westminster to develop a new approach to relations with the devolved governments, noting that Scotland and Wales are now led by nationalist parties and that relations between the four governments have become increasingly complex.

The Cardiff declaration is therefore likely to be closely watched in Westminster. While it would not itself create a legal right to hold an independence referendum, it would represent a significant political statement from the leaders of three of the UK’s four constituent nations.

For the moment, the UK Government remains opposed to granting Scotland an immediate route to another independence vote. The SNP, Plaid Cymru and Sinn Féin, however, are increasingly presenting national self-determination as a shared democratic principle rather than an issue confined to any one part of the United Kingdom.

Monday’s meeting could consequently mark a new stage in the constitutional debate — with the leaders of Scotland, Wales and Northern Ireland attempting to put their demands for greater self-government at the centre of the UK’s political agenda.