Northern Irish loyalist paramilitary groups have told British Prime Minister Boris Johnson they are temporarily withdrawing support for the 1998 peace agreement due to concerns over the Brexit deal.
While the groups pledged "peaceful and democratic" opposition to the deal, such a stark warning increases the pressure on Johnson, his Irish counterpart Micheál Martin and the European Union over Brexit.
Northern Ireland's 1998 peace deal, known as the Belfast or Good Friday Agreement, ended three decades of violence between mostly Catholic nationalists fighting for a united Ireland and mostly Protestant unionists, or loyalists, who want Northern Ireland to stay part of the United Kingdom.
The loyalist paramilitaries including the Ulster Volunteer Force, Ulster Defence Association and Red Hand Commando said they were concerned about the disruption to trade between Great Britain and Northern Ireland due to the Brexit deal.
"The loyalist groupings are herewith withdrawing their support for the Belfast Agreement," they said in a letter to Johnson quoted by the Belfast Telegraph.
They said they would not return to the deal until their rights were restored and the Northern Irish Protocol - part of the 2020 Brexit Treaty - was amended to ensure unfettered trade between Britain and Northern Ireland.
But, they said, their core disagreement was more fundamental: that Britain, Ireland and the European Union had in the Northern Irish Protocol breached their commitments to the 1998 peace deal and the two communities.
"Please do not under-estimate the strength of feeling on this issue right across the unionist family," the letter said.
"If you or the EU is not prepared to honour the entirety of the agreement then you will be responsible for the permanent destruction of the agreement," the letter said.
Preserving the delicate peace in Northern Ireland without allowing the United Kingdom a back door into the EU's markets through the 310-mile (500 km) UK-Irish land border was one of the most difficult issues of the Brexit divorce talks.
The loyalist groups abandoned the armed struggle in 1998 and residual violence since the accord has largely been carried out by dissident nationalist groups who opposed the peace deal.
Since Brexit proper on Jan. 1, 2020, Northern Ireland has had problems importing a range of goods from Britain - which unionists, or loyalists, say divides up the United Kingdom and so is unacceptable.
The European Union promised legal action on Wednesday after the British government unilaterally extended a grace period for checks on food imports to Northern Ireland, a move Brussels said violated the terms of Britain's divorce deal.
The EU's finance chief said Britain's move raised questions over whether it can be trusted in future trade negotiations with any partner.
"It does open a question mark about global Britain, if this is how global Britain will negotiate with other partners. Our experience has been not an easy one to put it mildly," Mairead McGuinness, who is negotiating post-Brexit financial services terms with Britain, told Irish broadcaster RTE on Thursday.
Joe Biden, while campaigning in the presidential election last year, bluntly warned Britain that it must honour the 1998 peace agreement as it withdrew from the European Union or there would be no separate U.S. trade deal.
Johnson's office did not immediately comment on the letter.