The Northern Ireland Protocol was lawfully enacted and must take precedence over a centuries-old legislative clause on trade with Britain, the Court of Appeal ruled today.
Senior judges dismissed a pan-unionist challenge to the post-Brexit trading arrangement after finding that it subjugates part of the Act of Union 1800, based on the sovereign will of Britain's parliament.
Northern Ireland's constitutional status within the UK remains unchanged, they held.
Chief Justice Dame Siobhan Keegan stated: "This case is very far from one where a court would even begin to contemplate whether it could intervene as the appellants suggest."
Implemented at the start of 2021 to prevent a hard Irish border, the Protocol means Northern Ireland remains in the EU single market for goods.
The new trade checks on produce entering the region from Britain has created a de facto customs border in the Irish Sea.
Amid widespread unionist opposition to the accord, a coalition including TUV leader Jim Allister, Kate Hoey and former Brexit Party MEP Ben Habib mounted a legal bid to have it declared unlawful.
In June last year, a High Court judge found that the Withdrawal Agreement Act, which introduced the Protocol, conflicts with Article 6 of the Act of Union, drawn up to ensure equal trade footing between Britain and Ireland.
However, he ruled that the new legislation overrides older law which cannot obstruct the clear specific will of parliament.
Those findings were appealed on the basis that the Act of Union has legal supremacy, with no power for the implied repeal of a constitutional statute.
The appellants argued that a legislative prohibition on any trade disparity with Great Britain renders the Protocol invalid.
But the court found that Article 6 of the Act of Union has been modified by the terms of the Withdrawal Agreement Act - without any implied repeal.
"Parliament was clearly sighted on the Protocol which was the end result of a protracted, transparent, debated, informed and fully democratic process which decided arrangements for Northern Ireland post Brexit," Justice Keegan said.
"The terms were settled and made law after a long parliamentary process, and it cannot be seriously suggested that Parliament was unaware of the changes that may be wrought."
With previous Acts of Parliament subject to the Withdrawal Act, she added: "This means that the terms of the Protocol take precedence."
Further grounds of challenge based on claims that the Protocol is incompatible with the 1998 Northern Ireland Act and breaches the European Convention on Human Rights were also rejected.
With a separate challenge by Belfast man Clifford Peeples similarly dismissed, Justice Keegan confirmed: "We have determined that none of the legal arguments presented by the appellants prevail.
"Accordingly, the appeal must be dismissed."
