British Prime Minister Andy Burnham "believes strongly in the union," his spokesperson said Monday in a first reaction to a self-determination declaration signed by the first ministers of Scotland, Wales, and Northern Ireland.

"The prime minister is committed to delivering change across all four nations of the United Kingdom and believes strongly in the union," Burnham's spokesperson was quoted by Sky News as saying.

The spokesperson told reporters that the UK is "at its best" when people come together around shared values and problem-solving, "rather than division."

"The prime minister will continue to ease pressures on household finances, and delivering greater economic security, rather than engaging in constitutional debates or further referendums."

The statement came after the leaders of Scotland, Northern Ireland, and Wales earlier Monday signed a landmark memorandum of understanding, urging the UK government to prepare for constitutional change.

A joint declaration signed by Rhun ap Iorwerth, first minister of Wales and leader of Plaid Cymru, John Swinney, first minister of Scotland and leader of Scottish National Party (SNP), and Michelle O'Neill, first minister of Northern Ireland and vice-president of Sinn Fein party, said the political landscape "is changing."

"Each of our nations has the right to determine its own future, through democratic means and in accordance with the wishes of its people," the document noted.

The declaration came following the first face-to-face meeting between the three nationalist leaders in Wales, the Welsh capital, earlier on Monday.

For the first time since devolution began in 1997, all three nations have first ministers who want to leave the UK.

It came after US President Donald Trump said Sunday that integrating Ireland and Northern Ireland represents "one of the naturals of all time," suggesting historical partitions should be resolved.

The remarks followed comments he made Saturday while in Ireland, when he said he would "love" to see a united Ireland. (AA)