The Northern Ireland Protocol is lawful, the UK Supreme Court has ruled.
It reaffirmed the findings of two lower courts, which had dismissed a legal challenge by a group of unionists.
TUV leader Jim Allister and others argued that the protocol was incompatible with the 1800 Act of Union between Britain and Ireland and 1998 legislation that enacted the Good Friday Agreement.
The case had been rejected by both the High Court and the Court of Appeal in Belfast and had been appealed to the Supreme Court.
Today, it also dismissed the appeal on all grounds.
The lower courts said Westminster's decision to pass the Withdrawal Act, which contains the protocol arrangements, meant the Acts of Union had been lawfully modified.
They also rejected arguments that Northern Ireland's constitutional status had been changed by the new arrangements.
The judges found that the Secretary of State for Northern Ireland did have the power to change a future Stormont Assembly vote on the continuation of the protocol from a cross-community one, to one needing only a simple majority.
Announcing the court's decision, Lord Stephens said the appeal was "unanimously dismissed" on all grounds.
He said in the ruling: "The most fundamental rule of UK constitutional law is that parliament, or more precisely the crown in parliament, is sovereign and that legislation enacted by parliament is supreme.
"A clear answer has been expressly provided by parliament in relation to any conflict between the protocol and the rights in the trade limb of article VI (of the Acts of Union 1800)."
DUP leader Jeffery Donaldson said the case had highlighted why unionists are opposed to the trading arrangements.
Mr Donaldson, who attended the judgment hearing in London, said: "A solution to the protocol was never going to be found in the courts, but the cases have served to highlight some of the reasons why unionists have uniformly rejected the protocol.
"The government must consider this judgment, their own arguments to the court and take the steps necessary to replace the protocol with arrangements that unionists can support.
"The protocol represents an existential threat to the future of Northern Ireland's place within the Union. The longer the protocol remains, the more it will harm the Union itself."
Former Brexit Party MEP Ben Habib, one of those who brought the legal challenge, said the Supreme Court judgment provided "constitutional clarity" that the Good Friday Agreement was "broken".
Reacting to the court judgment, Mr Habib told the BBC: "The two key takeaways are that a significant part of the Acts of Union have been disapplied, contrary to all the various protestations made by parliamentarians.
"And the cross-community consent, which is a cornerstone of the Good Friday Agreement, has also been disapplied, in order to get the protocol through.
"We have complete constitutional clarity and that is itself is a victory for ourselves.
"We have just had the Supreme Court confirm there is an Irish Sea border and it was imposed without cross-community consent and that is what we have been arguing for.
"They may have ruled that it has legally done so but effectively the United Kingdom of Britain and Northern Ireland is broken, the Good Friday Agreement is broken and that is complete constitutional clarity for us."