The DUP and other unionists are to mount a legal challenge to the Northern Ireland protocol, challenging its "compatibility with Act of Union 1800, the Northern Ireland Act of 1998 and the Belfast Agreement", DUP leader Arlene Foster has announced.
It is understood that the DUP sought the legal opinion of constitutional law experts ahead of several potential High Court challenges in both Belfast and London against the British government over post-Brexit Irish Sea trading arrangements.
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.
A party source involved in the initiative told PA that preparatory work on a "series of very significant legal challenges" is at an "advance stage".
"No stone will be left unturned in the pursuit of justice for the people of the Union," the source said.
While the shape of the potential challenges has not yet been detailed, many unionists have argued that the protocol undermines both the Act of Union and the Northern Ireland Act, which gives legislative effect to the Good Friday/Belfast Agreement.
Another potential legal challenge against the Government over the terms of the protocol is being threatened by peer Baroness Kate Hoey, Traditional Unionist Voice leader Jim Allister and former Brexit Party MEP Ben Habib.
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 tomorrow.
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 Northern Ireland from Great Britain.
This has led to some trading disruption since the end of the Brexit transition period on 31 December.
It has also heightened political tensions in Northern Ireland, with unionists enraged with arrangements they claim drive an economic wedge between the region and the rest of the UK, undermining the constitutional integrity of the Union as a consequence.
Another layer of protocol red tape comes into effect on Monday, when health certifications will be required on sausages and other chilled meats entering Northern Ireland from Great Britain.