A two-day hearing examining the lawfulness of the Northern Ireland Protocol is underway at the UK's highest court.
The Supreme Court is considering an appeal against previous court decisions to reject a judicial review into the legality and constitutionality of post-Brexit trading arrangements.
The challenge has been brought by a group of unionists, including TUV leader Jim Allister, Brexit Party MEP Ben Habib and Baroness Kate Hoey.
They contend that the Protocol is incompatible with the Act of Union of 1800 and the Northern Ireland Act that underpins the Good Friday Agreement.
The case was rejected by a judge at the High Court in Belfast last year who effectively ruled that the protocol has primacy over these other acts.
An appeal of that decision was dismissed earlier this year.
But the Court of Appeal did point to issues for the Supreme Court to consider.
Opening arguments were presented on Wednesday by John Larkin KC on behalf of those taking this case.
He asked whether the UK Parliament, in creating the Withdrawal Act that brought about the protocol, intended to cause fundamental change to the constitution.
Mr Larkin KC said the Acts of Union "continue to provide the essential structure of the UK" and that it "can't be repealed by general words".
He argued that part of the protocol "speak with a forked tongue" when it comes to NI's constitutional status.
Mr Larkin KC said the protocol had "fundamentally changed" Northern Ireland's position within the UK, in a way that was "incompatible" with the Northern Ireland Act.
The hearing is due to conclude on Thursday, with a ruling at a later date.
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