DUP leader Arlene Foster and senior party MPs are launching legal action challenging the Northern Ireland Protocol.
They will be joining other unionists from across the UK in judicial review proceedings unless alternative post-Brexit 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 Mrs Foster’s action in response to disruption of business through Irish Sea ports.
Fundamental to the Act of Union is unfettered trade throughout the UK
Mrs 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.”
Mrs 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.
Mr 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.
A party source involved in the initiative told the PA news agency that preparatory work on a “series of very significant legal challenges” is at an “advanced stage”.
“No stone will be left unturned in the pursuit of justice for the people of the Union,” the source said.
Unionists have argued that the protocol undermines the Act of Union and the Northern Ireland Act, which gives legislative effect to the 1998 Good Friday/Belfast Agreement which established devolved powersharing.
That campaign includes a boycott of North-South ministerial engagement on issues related to the contentious trading arrangements.
The party also initiated an online petition to secure a parliamentary debate on the protocol – the debate is due to take place at Westminster on Monday.
The protocol was agreed by the EU and UK to overcome one of the main sticking points in the Brexit withdrawal talks – the Irish border.
It keeps that frontier free flowing by Northern Ireland remaining in the single market for goods and applying EU customs rules at its ports.
The protocol instead moved the regulatory and customs border to the Irish Sea, with a series of checks, certifications, inspections and declarations now required on many goods being shipped into the region from Great Britain.
This has led to some trading disruption since the end of the Brexit transition period on December 31.