Arlene Foster has launched a legal challenge to the Northern Ireland protocol which she said has driven "a coach and horses" through the Act of Union.
The DUP leader joins other unionists from across the UK in launching 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.
A separate group of DUP members has also engaged senior legal counsel to prepare for a series of challenges to the protocol.
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."
Taoiseach Micheal Martin has urged the DUP to dial down the rhetoric and put politics aside.
Mrs Foster is joining the legal challenge by peer Baroness Kate Hoey, Traditional Unionist Voice leader Jim Allister and former Brexit Party MEP Ben Habib.
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 power sharing.
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.