The DUP has announced that its MPs will vote against a key component of the UK Government's Windsor Framework in British parliament this week.
DUP leader Jeffrey Donaldson said the party's MPs will vote against the "Stormont Brake" aspect of the framework, which aims to give the Northern Ireland Assembly a greater say on how new EU laws apply to Northern Ireland.
The UK parliament is set to vote on the Stormont Brake on Wednesday.
The measure would allow the British Government to stop or delay new EU laws from applying to goods in Northern Ireland if so requested by at least 30 MLAs from at least two parties in the region.
There are currently 35 MLAs from unionist parties in the Northern Ireland Assembly, which has a total of 90 MLAs. Meanwhile, two independent MLAs also identify as unionists.
UK Prime Minister Rishi Sunak said the Stormont Brake was proof that the UK has "taken back control" in negotiations with the EU. A spokesperson for Sunak also said the measure was the "most significant part" of the Windsor Framework.
However, Donaldson said on Monday that the DUP will be voting against the Stormont Brake, citing "fundamental problems" with the Windsor Framework.
Donaldson said in a statement that "key areas of concern" remain that require further clarification, re-working, and change.
He added that Wednesday's vote could be seen as a vote on the entire framework and not just on the Stormont Brake.
"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 on Monday.
"Since the announcement that the "Stormont Brake" is to be debated and voted upon in Parliament on Wednesday, there have been a number of indications that this vote will be read as indicative of current positions on the wider Windsor Framework package.
"We will continue to work with the government on all the outstanding issues relating to the Windsor Framework package to try to restore the delicate political balances within Northern Ireland and to seek to make further progress on all these matters."
The DUP has refused to enter power-sharing with Sinn Féin since last May's assembly elections in Northern Ireland, citing concerns over the Northern Ireland Protocol, a post-Brexit trade measure that aimed to avoid a hard border on the island of Ireland by introducing some customs checks for goods arriving in Northern Ireland from Great Britain.
A majority of MLAs who were elected during last year's elections are in favor of the Protocol remaining in some form.
Unionist MLAs claim that the Protocol threatens Northern Ireland's position within the United Kingdom and want it replaced with new measures.
Rishi Sunak has announced that the proposed Windsor Framework will "remove any sense of a border in the Irish Sea" and protect Northern Ireland's position in the UK.
Sunak said the Windsor Framework will ensure a smooth flow of trade within the UK, with goods destined for Northern Ireland arriving through a green lane and goods destined for Ireland and the EU arriving through a red lane.
"In the Green Lane, burdensome customs bureaucracy will be scrapped," Sunak said at the end of February.
"It means food retailers like supermarkets, restaurants, and wholesalers will no longer need hundreds of certificates for every lorry."