DUP leader Arlene Foster and senior party MPs including Sir Jeffrey Donaldson and Sammy Wilson are backing legal action against the Northern Ireland Protocol.
They will be joining other likeminded unionists from across the United Kingdom as named parties in judicial review proceedings challenging the protocol’s compatibility with the Act of Union 1800, the Northern Ireland Act of 1998 and the Belfast Agreement.
Mrs Foster said: “Fundamental to the Act of Union is unfettered trade throughout the United Kingdom.
“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.”
Traditional Unionist Voice (TUV) leader Jim Allister is also taking part in judicial review action against the Northern Ireland Protocol.
He 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 and particularly Article 6 thereof.”
Ms Foster has urged the British government to deal with the “rupturing” of the east-west relationship caused by the Northern Ireland Protocol.
On Monday a debate at Westminster triggered by her party’s e-petition will ask the British government to take action to replace this “flawed and deeply damaging” Protocol.
The DUP has vowed to overthrow the post-Brexit trade mechanism which has disrupted trade from the rest of the UK to Northern Ireland.
Mrs Foster said: “There is no question of the public anger towards the Protocol. The petition demonstrated that.
“Brussels and London must work for a permanent solution. Temporary measures will not work.
“The Protocol is flawed and has ruptured the east-west strand of the Belfast Agreement.”
On Monday, Parliament will debate the e-petition she tabled calling for the triggering of Article 16 which allows the British government to override the protocol and ensure unfettered trade from GB-NI.
More than 140,000 have signed the petition, with the 100,000-signature threshold for a debate surpassed in just over 24 hours.
Mrs Foster said: “The European Union and the Government must recognise that to press on with the Protocol with every single unionist party opposed to it will lead to failure.
“Every agreement involving Northern Ireland has always required the consent of all sides. Consensus has been our watchword.”
The EU moved to trigger Article 16 over the distribution of vaccines but quickly reversed its decision.
Mrs Foster said: “The European Union claim their triggering of Article 16 was ‘a mistake’ but to close their eyes to unionist opposition to this Protocol will be another mistake.
“The Prime Minister must work for a permanent solution before the end of the grace periods when even greater disruption will become apparent.
“There needs to be unfettered trade from GB-NI.”