One of the key architects of the Good Friday Agreement has united with unionist leaders in threatening a legal challenge against the Government over the Northern Ireland protocol.
Lord Trimble, the former first minister of Northern Ireland, has joined forces with Arlene Foster and a cross-party group threatening a legal challenge over the arrangements governing post-Brexit trade in the province.
Speaking to The Telegraph on Tuesday, Lord Trimble, who was awarded the Nobel Peace Prize for his role in the Belfast Agreement, confirmed that he would join the action if it ended up in court.
In a letter sent to the Prime Minister, the group claims the protocol is causing "immense" economic damage and is "nothing less than vassalage".
Warning of the equally "pernicious constitutional damage" it has caused, they add they will make good on their intention to bring a judicial review unless "you take immediate action to settle a new arrangement for Northern Ireland."
It was coordinated by Ben Habib, a former Brexit Party MEP on behalf of Lord Trimble; Mrs Foster and three senior DUP leaders; Steve Aiken, the leader off the Ulster Unionist Party; Jim Allister, the leader of Traditional Unionist Voice; and Baroness Hoey, a crossbench peer.
It comes just hours before Michael Gove is due to resume crunch talks with his EU counterpart Maros Sefcovic today as part of the UK-EU joint committee, which presides over the arrangements for Northern Ireland.
While Mr Johnson has said he is prepared to unilaterally override the protocol if the EU fails to compromise, ministers hope to resolve the problems under the existing agreement.
Ahead of the talks, Mr Sefcovic said the EU would be open to "pragmatic and flexible solutions" to problems with the protocol, but reiterated that it was a "two way street" and the UK would be expected to step up checks at the border.
He also warned that "not everything can be solved", adding that there were "inevitable consequences of Brexit."
On Tuesday, Mr Gove also confirmed that the UK would agree to extend the deadline for approving the Brexit trade deal until April, after the bloc requested more time for it to be ratified.
The European Commission chose to provisionally apply the treaty before January due to it being struck at the last-minute, but must now make sure it is fully ratified.
On Sunday, The Telegraph revealed that the legal challenge had been threatened over the protocol, which was established to smooth trade issues resulting from the province remaining in the UK internal market while continuing to apply many EU rules, but is now blamed for causing significant disruption.
While the Government is seeking to extend a number of grade periods for supermarkets, parcel couriers, chilled meat products and medicines until 2023, the group claims this "will do nothing to ameliorate the Protocol's fundamental assault on our constitutional position."
They also argue that the protocol flies in the face of the Act of Union 1800 and the Good Friday Agreement, and have challenged the way in which Northern Ireland will be asked to provide consent for the protocol continuing or ending in 2024.
They have instructed John Larkin QC, the former attorney general of Northern Ireland, to seek a judicial review, and have issued a letter before action.
The letter states: "By placing Northern Ireland in a foreign single market for goods, subject to a foreign customs code and VAT regime, all governed by foreign laws we did not make and cannot change and overseen by a foreign court, this part of the United Kingdom has experienced a transfer of sovereignty which amounts to a fundamental constitutional change.
"lt is estimated that 60 per cent of the laws that govern our economy are now made not in Belfast, or London, but in Brussels.
"This is nothing less than vassalage and self-evidently wholly incompatible with Northern Ireland's constitutional position as an equal and integral part of the United Kingdom.
"This has occurred without even a shred of consent from the people of Northern lreland, compounding the great wrong.
"As pro-unionists and leaders in the community in Northern Ireland we wish you to be in no doubt that our unified and unalterable position is that the Protocol is incapable of being reformed and must be repudiated and superseded by arrangements which fully respect Northern Ireland's position as a constituent and integral part of the United Kingdom."
The Government argues that its approach is in keeping with the Good Friday Agreement.
Speaking ahead of Wednesday's joint committee meeting, Mrs Foster said: "Northern Ireland is suffering real economic and societal difficulties as a consequence of the Northern Ireland Protocol operating and creating new barriers to unfettered trade within the United Kingdom and disrupting supply lines of goods to Northern Ireland.
"Our Government should use all the powers it has to move urgently to protect UK trade and to ensure all UK goods and produce can freely flow to and from every part of the United Kingdom."