The first ministers of Wales, Scotland and Northern Ireland have declared that "Westminster's time is running out" and that the British government must "prepare" for constitutional changes, the BBC reports.
The nationalist leaders of "Plaid Comrie", "Sinn Fein" and the Scottish National Party (SNP) — who want their countries to leave the UK — today signed an agreement stating that no government in Westminster has the right to "block democracy".
However, while Scottish leader John Sweeney and Northern Ireland leader Michelle O'Neill called for referendums at a joint press conference, Welsh leader Rhoon ap Iorweth refused to give a deadline for holding such a poll in his country.
The meeting was also attended by the chairwoman of "Sinn Fein" Mary Lou McDonald.
App Iorweth will hold further talks with Sweeney this afternoon in their capacity as heads of government, with the two expected to sign an agreement for "future cooperation in areas of mutual interest".
For the first time since the start of the devolution process, Wales, Scotland and Northern Ireland have first ministers who want their countries to leave the United Kingdom.
The three met today as representatives of their respective parties.
The memorandum of understanding reads: "For those watching from these islands — and for those watching from around the world — There could be no clearer sign that Westminster's time is running out.
No government in Westminster has the right to block democracy or undermine the principle that our peoples will determine their own future.
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.
A spokesman for the Plaid Cymru party said before the meeting that the "changing political landscape in these islands" was a strong case for "practical political cooperation" in areas such as energy, the economy and engagement with Europe.
He said this included "the wider political and constitutional issues facing our nations, as well as the right to self-determination".
Ap Iorweth noted that the Welsh Assembly elections in May were "a turning point for Wales" and that "there is a growing recognition that the Westminster model of government no longer works".
Plaid Comrie had previously ruled out holding an independence referendum before the next Senedd election in 2030.
SNP leader Sweeney said it was "a significant and turning point for the United Kingdom" and that things have "changed forever".
"The people of Scotland, Wales and Ireland have the right to self-determination," he said. "They have the right to decide their own future".
The deputy leader of "Sinn Fein" O'Neill said the interests of the people of the entire island of Ireland "have never been protected by those in power in Westminster". "But change is happening across these islands and we must be ready for constitutional change when it comes", she warned. "The election of three nationalist first ministers for the first time in these islands is a clear sign that people see that Westminster is not working in the interests of the people of Northern Ireland, Scotland or Wales". Sinn Féin leader Mary Lou McDonald said the paths to self-determination for Wales, Scotland and Ireland were different, describing the current period as a "historic moment". "We obviously have different national histories, but we have one very important thing in common - "our firm insistence that it is the people of Ireland who must decide the future of Ireland, the people of Scotland — the future of Scotland, and the people of Wales — the future of Wales," she said.
The meetings came just days after the issue of Scottish independence was raised again in the House of Commons.
During parliamentary questions to the Prime Minister, Andy Burnham suggested that a second referendum could be held in Scotland if there was clear public support for such a vote.
He compared the situation to the conditions for a referendum on the border in Northern Ireland, explaining that the Good Friday Agreement clearly states that a referendum should be "considered by the Secretary of State for Northern Ireland where there is a clear consensus in the country, where public opinion has changed to the extent that the request should be considered".
"I would say the situation in Scotland is exactly the same", he added.
The apparent softening of tone was welcomed by Sweeney, before Burnham clarified that another referendum on Scottish independence remained "out of the question".
In the Scottish independence referendum in September 2014, the country voted to remain part of the United Kingdom by 55% to 45%.
But the political situation in Wales is different to that in Scotland.
Ap Iorweth, who became the first non-Labour leader of Wales after the Senedd election in May, ruled out pushing for a referendum on Welsh independence during his party's first term in government. He said he wanted to have a "national conversation" on the issue.
His government intends to set up a national commission to "lay the groundwork" for future plans for Welsh independence.
Meanwhile, Welsh ministers are pushing for more powers to be transferred to Wales, including in the areas of policing and juvenile justice.
Burnham said he was "open" to the idea.
Before becoming prime minister, Burnham was a proponent of change, but on a recent visit to Wales he played down the prospect of reform.
Leaders of the devolved administrations will meet the prime minister next month.
The British government said: "The prime minister is committed to delivering change across all four nations of the United Kingdom and firmly believes in the union.".