The DUP has heavily criticised the new Brexit deal, claiming it will damage Northern Ireland’s economy and undermine the Union.
The party said the proposals for democratic consent, with Stormont voting on a straight majority basis, drive a coach and horses through the Good Friday (Belfast) Agreement.
In a lengthy statement, the Conservatives’ confidence-and-supply allies made clear they would not support Boris Johnson’s deal at Saturday’s crucial Westminster vote.
“We have been consistent that we will only ever consider supporting arrangements that are in Northern Ireland’s long-term economic and constitutional interests and protect the integrity of the Union,” the party said.
“These proposals are not, in our view, beneficial to the economic well-being of Northern Ireland and they undermine the integrity of the Union.
“Our main route of trade on an East-West basis will be subject to rules of the European Union customs union, notwithstanding that Northern Ireland will remain part of the UK customs territory.
“All goods would be subject to a customs check regime regardless of their final destination.
“The default position, even for goods travelling from one part of our country to another, is that they are considered under the EU customs code unless otherwise agreed.
“We recognise that only those goods ultimately destined for the Republic of Ireland would be subject to tariffs but the reality remains that the EU would have a veto on which goods would be exempt and which would not under the Joint Committee arrangements.
“This is not acceptable within the internal borders of the United Kingdom.
“Consumers in Northern Ireland would face the prospect of increased costs, and potentially less choice, due to checks being implemented in order to facilitate the European Union.
“Throughout all the discussions on these issues, we have been clear that Northern Ireland should not be subjected to administrative burdens which will be entrenched for the future.
“On VAT, Northern Ireland will again be bound into arrangements that the rest of the United Kingdom will not.
“There is a real danger that over time Northern Ireland will start to diverge across VAT and customs and without broad support from the democratic representatives of the people of Northern Ireland.”
The party said “some progress” had made been made on the issue of giving Northern Ireland a democratic say on post-Brexit arrangements.
However, it criticised the fact that Stormont would not be given a vote on whether to implement the arrangements at the outset, only after they have been in operation for four years.
The party also voiced concern that the vote in 2025 would be conducted on a straight majority basis, not using the peace process voting mechanism that requires the consent of both a majority of unionists and a majority of nationalists.
“These arrangements would be subject to a rolling review but again the principles of the Belfast Agreement on consent have been abandoned in favour of majority rule on this single issue alone,” the DUP’s statement added.
“These arrangements will become the settled position in these areas for Northern Ireland.
“This drives a coach and horses through the professed sanctity of the Belfast Agreement.”
The statement concluded: “For all of these reasons it is our view that these arrangements would not be in Northern Ireland’s long-term interests.
“Saturday’s vote in Parliament on the proposals will only be the start of a long process to get any Withdrawal Agreement Bill through the House of Commons.”