Leaders of Scotland, Wales and Northern Ireland are set to sign a joint declaration calling for the right to determine their own political future, including the possibility of leaving the United Kingdom.

The leaders are expected to meet in Cardiff, the capital of Wales, on Monday to discuss the future of the union and growing calls for independence, The Telegraph, a British newspaper, reports.

The United Kingdom comprises four countries: England, Scotland, Wales and Northern Ireland. London, the UK capital, is in England.

Scotland’s First Minister John Swinney, Wales’ First Minister Rhun ap Iowerth and Northern Ireland’s First Minister Michelle O’Neill are expected to sign a memorandum of understanding (MOU) aimed at starting discussions on the countries’ right to self-determination.

The development comes amid renewed debates over Scottish independence and Irish reunification.

Mr Swinney described the current political arrangement as unsustainable and called on Westminster, the UK government, to prepare for a future beyond the union.

“For the first time, Scotland, Wales and Northern Ireland are led by pro-independence first ministers. There could be no clearer sign that the status quo is not sustainable – and that Westminster must prepare for a post-UK future,” he said.

He added that the meeting in Cardiff would focus on discussing a better future for the countries, arguing that independence offers a fresh start.

The planned declaration comes a day after US President Donald Trump expressed support for the reunification of Ireland.

Speaking during a visit to Dublin on Saturday, Mr Trump said he would like to see a united Ireland.

He described the possibility of Northern Ireland joining the Republic of Ireland as “a great thing.”

Northern Ireland remains part of the United Kingdom, while the Republic of Ireland is an independent country.

The Cardiff meeting is also expected to push for another Scottish independence referendum.

Last Wednesday, UK Prime Minister Andy Burnham said he would authorise a fresh referendum if there was sufficient public demand.

The leaders’ meeting is expected to increase pressure on the UK government to approve the vote.

Scotland held its last independence referendum in 2014, when voters rejected independence and chose to remain in the United Kingdom.

England and Scotland formally united in 1707 to create Great Britain. Ireland joined the union in 1801.

Most of Ireland became independent in 1922, but Northern Ireland remained part of the United Kingdom.