DUP leader Arlene Foster and senior party MPs are launching legal action challenging the post-Brexit agreement surrounding the Northern Ireland Protocol.
They will be joining other unionists from across the UK in judicial review proceedings unless alternative trade arrangements are put in place which secure their consent.
DUP deputy leader Nigel Dodds, the party’s Westminster leader Sir Jeffrey Donaldson and chief whip Sammy Wilson are backing Foster’s action in response to disruption of business through Irish Sea ports.
A separate group of DUP members has also engaged senior legal counsel to prepare for a series of challenges to the protocol.
Foster said: “Fundamental to the Act of Union is unfettered trade throughout the UK.
“At the core of the Belfast Agreement was the principle of consent yet the Northern Ireland Protocol has driven a coach and horses through both the Act of Union and the Belfast Agreement.”
The move comes amid ongoing unionist and loyalist anger at new regulatory and customs processes required to bring goods into Northern Ireland from the rest of the UK.
Nationalists and the Irish government are committed to solving problems with the protocol keeping Northern Ireland within the EU’s single market but insist nothing must threaten the free-flow of commerce on the island of Ireland.
Nationalist SDLP leader and Foyle MP Colum Eastwood said: “The DUP’s legal action against the Ireland Protocol is ill-judged and will only further entrench the febrile political environment as well as creating further uncertainty for people and businesses.
“There will be few with sympathy for the argument that the protocol, which prevents a hard border in Ireland and guarantees dual market access for local businesses, breaches the Good Friday Agreement.”
Taoiseach Micheal Martin has urged the DUP to dial down the rhetoric and put politics aside.
Foster said: “Neither the Northern Ireland Assembly, the Northern Ireland Executive nor the people of Northern Ireland consented to the protocol being put in place or the flow of goods from Great Britain to Northern Ireland being impeded by checks.
“They certainly did not consent to the arrangements for those checks being determined by a power over which we have no democratic say.”
They are joining the legal challenge by peer Baroness Kate Hoey, Traditional Unionist Voice leader Jim Allister and former Brexit Party MEP Ben Habib.
Allister said: “Whereas the protocol is doing immense damage to commerce and thus our economy, it is its pernicious constitutional damage which concerns us above all.
“Extended ‘grace periods’, derogations and easements will do nothing to ameliorate the protocol’s fundamental assault on our constitutional position.
“At the heart of this challenge will be the irreconcilable conflict of the sovereignty-busting protocol with the foundational constitutional statute which bound us into the UK, the Act of Union 1800.”
The Ulster Unionists are also challenging the protocol.
Leader Steve Aiken said: “The very cornerstone of our democracy stands on foundations based on an individual’s right to legally challenge what they believe to be unjust.”
The separate DUP group has sought the legal opinion of constitutional law experts ahead of several potential High Court challenges in Belfast and London against the government over the post-Brexit Irish Sea trading arrangements.