Northern Ireland's Democratic Unionist Party on Monday said it would vote against a key element of the British government's agreement with the European Union on post-Brexit trade rules, in a blow for British Prime Minister Rishi Sunak.
But leader Jeffrey Donaldson said his party might yet be convinced to support it in future if additional concessions are made.
Parliament will on Wednesday debate and vote on the so-called "Stormont brake", part of the Windsor Framework agreed in February in a bid to reset relations.
The brake enables Britain to prevent new EU laws applying to goods in Northern Ireland if asked to do so by a third of lawmakers in the province's devolved legislature. The DUP has complained that it does not apply to existing EU law.
Wednesday's vote is still likely to pass as the opposition Labour Party supports the overall agreement but the DUP's decision could increase the number of eurosceptic members of Sunak's own Conservative Party who refuse to back it.
The European Research Group (ERG) of pro-Brexit Conservative lawmakers is due to set out its verdict on Tuesday.
"Whilst representing real progress the 'brake' does not deal with the fundamental issue which is the imposition of EU law by the Protocol," Donaldson said in a statement, referring to the part of Britain's EU divorce deal that governs Northern Ireland trade.
"Our party officers...met this morning and unanimously agreed that in the context of our ongoing concerns and the need to see further progress secured whilst continuing to seek clarification, change and re-working that our Members of Parliament would vote against the draft statutory instrument."
The DUP has for a year boycotted Northern Ireland's power-sharing government over its opposition to post-Brexit trade rules, which effectively keep Northern Ireland in the EU's single market in order to avoid the need for a hard border on the island of Ireland.
Such a border would be seen as endangering the Good Friday Agreement which largely ended three decades of armed conflict in Northern Ireland involving militants seeking a united Ireland, "loyalists" wanting to remain part of the United Kingdom, and British security forces.
The new agreement was painstakingly hammered out by Sunak after then Prime Minister Boris Johnson's government threatened to renege on the orginal deal it had struck with the EU, saying it was unworkable.
Johnson was forced from office in September after a series of scandals and Sunak's retooled agreement was widely welcomed, including by the United States, which had said that any threats to the Good Friday Agreement could hurt the possibility of a US-UK trade deal.
Asked if the DUP might yet support Sunak's agreement if concessions are made, Donaldson told PA Media that "there's still some way to go".
He said UK legislation could prevent regulatory divergence between Northern Ireland and the rest of the United Kingdom caused by London dropping EU rules.
A spokesman for British Prime Minister Rishi Sunak said he was ready to reassure the DUP, but that no substantial changes to the agreement were planned.
Donaldson is also facing increasingly assertive opposition to compromise from within his own party and in the wider unionist community.
The small Traditional Unionist Voice (TUV) party, which has been a focal point for opposition to the proposed new rules, welcomed the DUP's decision on the vote.
"It is imperative that Unionism stands firm and makes it abundantly clear that the Windsor Framework is incapable of restoring UK sovereignty over NI and expunging EU colonial control," TUV leader Jim Allister said in a statement. (Reuters)