Plaid, SNP and Sinn Féin sign Cardiff co‑operation pact 2026

Key Points

• The Plaid Cymru, the SNP and the Sinn Féin concluded an agreement in Cardiff on Monday, 14 September 2026.
• The three leaders — Rhun ap Iorwerth, John Swinney, and Michelle O’Neill — called on the British government to “prepare for, plan and facilitate constitutional change” in Wales, Scotland and Northern Ireland.
• The document binds the parties to increased cooperation in the spheres of economic development, renewables and building relations with Europe and international players.
• “The Memorandum says that “no Westminster government has the right to obstruct democracy and the principle that our people will decide our future.”
• The leaders said that “Westminster’s days are over” and that “the momentum is with our cause and we believe constitutional change is coming.”

Cardiff (Wales Times) September 15, 2026 — Wales took centre stage in a rare tri‑nation summit as Plaid Cymru leader Rhun ap Iorwerth hosted Scottish National Party (SNP) leader John Swinney and Sinn Féin vice‑president Michelle O’Neill for talks on closer co‑operation, funding and constitutional change. The meeting, held on Monday, 14 September 2026, produced a signed memorandum of understanding that calls on Westminster to “prepare for, plan and facilitate constitutional change in each jurisdiction” and asserts that the peoples of Wales, Scotland and Northern Ireland have the right to determine their own futures.

What was agreed in the Cardiff memorandum of understanding?

The three parties set out a shared framework for practical co‑operation while reaffirming their distinct constitutional positions. As reported by multiple outlets, the memorandum states: “Our nations have the right to self‑determination. No Westminster government has the right to block democracy or undermine the principle that our people will decide their own future.” It adds: “We recognise that our parties have different constitutional positions and that each nation will determine its own future in its own way and in its own time.”

The text also declares: “For people watching across these islands – for people watching around the world – there could be no clearer sign that Westminster’s time is coming to an end,” and calls for the British government to “prepare for, plan and facilitate constitutional change in each jurisdiction.” The leaders further committed to “strengthening cooperation between our parties and identifying practical areas where working together can make a real difference.”

Which policy areas will the three parties work on together?

The memorandum identifies four primary areas for joint work: the economy, energy policy, European and international relations, and self‑determination. The parties pledged to share ideas on building “stronger and fairer economies”, expanding renewable energy resources, and rebuilding relationships with Europe, with the agreement stating that “the future of our nations belongs in the European Union.” They also said they would “look at how the parties can work constructively with one another” by “identifying practical areas of partnership around the economy, energy and European and international cooperation.”

What did Rhun ap Iorwerth, John Swinney and Michelle O’Neill say?

As reported by the BBC, the memorandum signed in uk/local/cardiff/">Cardiff included the line: “No Westminster government has the right to block democracy or undermine the principle that our people will decide their own future.” RTÉ reported that the leaders added: “We therefore commit to strengthening cooperation between our parties and identifying practical areas where working together can make a real difference.” The Guardian quoted the memorandum as saying: “Momentum is behind our movements; we believe that constitutional change is coming.”

On the wider political context, Nation.Cymru reported Mr Swinney as saying: “Crucially, for the first time since 2014, there is a British Prime Minister who has publicly accepted the democratic principle of self‑determination.” Axios noted that Northern Ireland’s first minister said recent comments by US president Donald Trump on a united Ireland showed the debate over the UK union had “gone global.”

How does the summit fit into wider UK constitutional debates?

The summit comes amid renewed pressure on Westminster over referendums and devolution. The Irish Times reported that first ministers were pushing for referendums despite UK prime minister Andy Burnham having “dashed hopes” on that front. The Guardian noted that the Cardiff declaration is likely to challenge Mr Burnham’s devolution agenda, with the three leaders calling for constitutional change in Wales, Scotland and Northern Ireland. ITV News Wales reported that the nationalist leaders agreed there is “no clearer sign that Westminster’s time is coming to an end” and committed to work together on areas including strengthening relationships with Europe.

Reports also highlighted that the issue of Scottish independence and Irish reunification resurfaced recently after Prime Minister Burnham wrote to Mr Swinney to reiterate that a repeat of the 2014 Scottish independence referendum remains “off limits,” following apparent misspeaking during prime minister’s questions. WION reported that the first ministers declared “Westminster’s time is coming to an end” and that the UK government should prepare for constitutional change.

What are the limits of the Cardiff agreement?

While the memorandum establishes a joint framework, it carries no legal authority to mandate independence referendums. The agreement instead sets out political commitments and areas for practical co‑operation, aiming to increase pressure on the UK government to recognise the right of people in the three nations to determine their constitutional futures. The text explicitly acknowledges that the parties hold “different constitutional positions” and that each nation will proceed “in its own way and in its own time.”

Background: how did this development come about?

The Cardiff summit marks the first face‑to‑face gathering of the three leaders since Rhun ap Iorwerth took office as Plaid Cymru leader and Welsh first minister in May 2026. It follows elections that left nationalist parties leading the devolved governments in Scotland, Wales and Northern Ireland simultaneously, creating a new alignment for cross‑border co‑operation. In the run‑up to the meeting, Plaid Cymru, the SNP and Sinn Féin signalled they would press for self‑determination votes and co‑ordinate on policy areas such as the economy, energy and Europe. The memorandum builds on earlier discussions about a “different attitude” from the UK government, with Mr ap Iorwerth saying the first ministers would discuss the “pursuit of a different attitude” from Westminster.

Prediction: how could this affect voters and businesses in Wales, Scotland and Northern Ireland?

If the Cardiff pact hardens into sustained policy co‑ordination, voters in the three nations may see more aligned messaging on constitutional options and greater emphasis on European links, potentially influencing how future elections are framed around self‑determination. For businesses, the focus on renewable energy and economic co‑operation could encourage cross‑border projects and investment pipelines that treat the three nations as a combined market for green energy and infrastructure, even as constitutional uncertainty persists. However, because the memorandum does not compel Westminster to grant referendums, the immediate effect is likely to be political pressure rather than an accelerated timetable for votes, meaning audiences should watch for subsequent moves by the UK government on devolution and funding.