The Democratic Unionist Party (DUP) will vote against the government’s Windsor Framework Brexit plans in parliament this week.
Its leader Sir Jeffrey Donaldson said he would continue to work with the government on “outstanding issues”.
MPs will be given a chance to vote on the so-called “Stormont Brake” aspect of the Windsor Framework on Wednesday.
Sir Jeffrey said the party had made the decision to vote against it during a meeting on Monday.
The framework builds on the Northern Ireland Protocol, which led to disagreements between the UK and European Union (EU) over trade rules.
The “Stormont brake” mechanism aims to give the Northern Ireland assembly a greater say on how EU laws apply to NI.
Prime Minister Rishi Sunak said it was proof that the UK has “taken back control” in the agreement he struck with the EU last month.
Last week, his spokesperson described the brake as the “most significant part” of the new deal.
More details on how it will work are being published later on Monday.
But Sir Jeffery said the DUP had “unanimously agreed” to vote against it because of “ongoing concerns”.
He told BBC News NI the party would continue to assess the deal, but that “we don’t believe that this represents the significant progress that we need to see in order to have the institutions restored at this point”.
He said the DUP’s view was that there “remain key areas of concern which require further clarification, re-working and change as well as seeing further legal text”.
‘The final lap’
“We’re looking to the government to ensure that there is further legislation that will protect Northern Ireland’s place within the United Kingdom and its internal market,” he added.
He said the party was committed to the restoration of the political institutions “under the right circumstances”.
“We haven’t come this far to falter in the final lap,” he added.
The DUP has blocked the functioning of the power-sharing government at Stormont for more than a year until its concerns about post-Brexit trading arrangements, agreed by the UK and EU, are addressed.
These arrangements, known as the Northern Ireland Protocol led to new checks being carried out on goods at Northern Ireland ports as some EU laws still applied in NI.
The checks were required under the original Brexit deal in order to maintain an open land border between Northern Ireland and the Republic of Ireland, which is in the EU.
A majority of members of the Northern Ireland Assembly elected in May 2022 are in favour of the Protocol, in some form, remaining.
Sinn Féin, Alliance and the SDLP have said improvements are needed to ease its implementation.
Unionist politicians want it replaced with new arrangements.
What is the Windsor Framework?
The Windsor Framework was signed to alter the Northern Ireland Protocol – and aims to significantly reduce the number of checks on any goods arriving in Northern Ireland from Great Britain.
The “Stormont Brake” mechanism would also allow the Northern Ireland Assembly to object to new EU rules.
Rishi Sunak previously said he was hopeful Stormont parties would “recognise that this a good deal and that will enable them to get back into a power-sharing executive”.
The DUP set up an eight-member panel to come to a “collective decision” on whether to support the framework.
Its co-operation is seen as vital if Stormont is to operate again.