A legislative prohibition on any trade disparity with Great Britain renders the Northern Ireland Protocol invalid, the Court of Appeal has heard.
Senior judges were told that the Acts of Union 1800 has a legal supremacy which denied power to introduce the post-Brexit arrangements.
Counsel for a group of unionists also argued that there can be no implied repeal of a constitutional statute.
John Larkin QC said: "The protocol could not be validly made because it is in conflict with a provision of the constitution of the United Kingdom – that is Article 6 of the Acts of Union."
TUV leader Jim Allister is heading the challenge to the accord implemented at the start of the year to prevent a hard Irish border.
Since the protocol came into force Northern Ireland has remained in the EU single market for goods.
But disruptions and checks on produce entering the region from Britain has created a de facto customs border in the Irish Sea.
The legality of those arrangements is disputed by a coalition including Baroness Hoey and former Brexit Party MEP Ben Habib, as well as former DUP and UUP chiefs Arlene Foster and Lord Trimble
They contend that the protocol unlawfully conflicts with both the Acts of Union 1800 and the 1998 Good Friday Agreement.
The case centres on the concept of implied repeal, where more recent legislation takes precedence over older law.
Earlier this year the High Court dismissed the pan-unionist bid to have the protocol declared unlawful.
A judge ruled that the Withdrawal Agreement Act, which introduced the Protocol, does conflict with the Acts of Union.
But he held that the new legislation overrode and effectively repealed relevant provisions within centuries-old law.
Written more than 200 years ago in an entirely different economic and political era, the Acts of Union cannot override the clear specific will of Parliament, the judge decided.
Appealing those findings, Mr Larkin argued that Article 6 of the Acts of Union prohibits any future law which leads to a trade disadvantage such as that created by the protocol.
"If the Protocol puts Great Britain and Northern Ireland on an unequal footing... then there was no power to make such a treaty," he submitted.
"The prerogative treaty-making power cannot, at least not since 1688, overcome the power of statute law."
Mr Larkin claimed the High Court judge's "primary error" was a failure to take into account the prohibitory clause within the older legislation.
"Protocol provisions that conflict with that obligation are not validly made. That is not validly agreed by Her Majesty's Government," he said.
Rejecting counter-arguments that any inconsistencies between the protocol and Article 6 are legally irrelevant, the barrister maintained "The Union does require an equal footing as to trade."
Tony McGleenan QC, for the Government, argued that the case centred on the principles of parliamentary sovereignty.
Detailing statutory developments since the 2016 Brexit referendum, he submitted that the provisions of the Withdrawal Act provided "a complete answer" to the appeal.
Parliament debated the process before being dissolved in 2019 and a General Election returning a Conservative administration with a manifesto to conclude the terms of the EU departure, counsel pointed out.
Mr McGleenan said: "This is where the answer to the case lies. It doesn't lie in abstract analysis of how competing constitutional statutes should be interpreted.
"Nor does it even lie in questions about the application of implied repeal, although they are informative.
"It lies in the words of the statute; this is what Parliament intended.
"The Acts of Union, all the other provisions that are in play are to be read and considered subject to the terms of the Withdrawal Agreement."
The appeal continues.
The hearing continues.