Northern Ireland’s main British unionist party says it will vote against a deal struck by Britain and the European Union to resolve a thorny trade dispute that vexed U.K.-EU relations

LONDON — Northern Ireland’s main British unionist party said Monday it will vote against a deal struck by Britain and the European Union to resolve a thorny trade dispute that vexed U.K.-EU relations and triggered a political crisis in Belfast.

Democratic Unionist Party leader Jeffrey Donaldson said the deal “does not deal with some of the fundamental problems at the heart of our current difficulties” and the party’s eight lawmakers would not support it when the House of Commons votes on Wednesday.

The U.K.-EU agreement, known as the Windsor Framework, aims to ease customs checks and other hurdles for goods moving to Northern Ireland from the rest of the U.K. that were imposed after Brexit to maintain an open border between the north and its EU neighbor the Republic of Ireland. The open border is a key pillar of Northern Ireland’s peace process.

Northern Ireland’s British unionist politicians strongly opposed the customs border, saying it undermines Northern Ireland’s place in the United Kingdom. The DUP walked out of the power-sharing semi-autonomous Belfast government a year ago in protest, leaving Northern Ireland’s 1.9 million people without a functioning administration.

The Windsor Framework is designed to ease the burden on businesses and address unionist concerns. It gives Northern Ireland politicians a mechanism, known as the Stormont Brake, to challenge new EU trade rules that could apply in the region — a key unionist demand.

But the DUP is concerned that the mechanism does not go far enough and some elements of EU law will continue to apply in Northern Ireland. Under the deal, Northern Ireland is part of the U.K.’s internal market but also has access to the EU’s vast single market of 27 nations for trade in goods.

The DUP decision is a blow to Prime Minister Rishi Sunak’s deal, but likely not a fatal one. Wednesday’s vote, on the Stormont Brake part of the deal, is a chance for lawmakers to express a view, but not essential to the framework being implemented.

The DUP also indicated it might change its mind in future. Donaldson said the party would seek “further clarification, re-working and change” and would “continue to work with the government on all the outstanding issues.”