Parliament will on Wednesday debate the so-called “Stormont brake”, a key part of an agreement that enables Britain to stop new European Union laws from applying to goods in Northern Ireland if so requested by a third of lawmakers in the province’s devolved legislature.

“I am categorically voting against and I would be surprised if my colleagues do not join me,” Ian Paisley Jr told the Belfast News Letter newspaper today.

While a vote on measures to implement the brake is likely to pass as the opposition Labour Party supports the overall agreement, the backing of the DUP is seen as crucial in British prime minister Rishi Sunak’s efforts to sell the deal to sceptical members of his own Conservative Party.

DUP leader Jeffrey Donaldson last week voiced his strongest concerns to date over the deal, saying his party was seeking changes from the British government, but he stopped short of promising to vote against the deal.

A spokesperson for the DUP declined to comment on how the party planned to vote after the debate, which he said was being framed by the government as an overall endorsement of the deal, known as the Windsor Framework.

The framework, he said, “does not deal with some of the fundamental problems at the heart of our current difficulties”.

“It is our current assessment that there remain key areas of concern which require further clarification, reworking and change as well as seeing further legal text,” the spokesperson said.

The BBC reported the party was due to meet today to agree a position ahead of the vote. The DUP did not immediately respond to a request to confirm the meeting.