There has been no constitutional change as a result of the Northern Ireland Protocol, a barrister for the UK government has told the Supreme Court.
Part of the Brexit deal, the protocol creates a trade border between Northern Ireland and the rest of the UK.
It is challenged by unionist politicians who say it breaches the Acts of Union and the Northern Ireland Act.
However Tony McGleenan KC said their arguments were “seriously overplayed.”
Mr McGleenan told the court “the idea that there has been a de facto termination of the union is not a tenable proposition”.
Earlier this year the Court of Appeal suggested the Supreme Court should examine whether a modification of the Acts of Union by the protocol amounted to a change in Northern Ireland’s constitutional status.
On Wednesday the applicants’ barrister told the court the Acts of Union were the “fundamental constitutional structure” of the UK.
John Larkin KC focused on the part of the Acts of Union which states Northern Ireland must be on an “equal footing” with Great Britain when it comes to trade.
“The protocol, taken as a whole and when one looks at some of its individual provisions in-depth, undoubtedly conflicts with Article 6 of the Acts of Union and we say this is acknowledged by the respondent,” Mr Larkin said.
Mr McGleenan said there had been “much misunderstanding” about what the protocol meant for the Acts of Union.
He said there was “a view abroad that the Act of Union has been repealed or subjugated”.
However he said Parliament had taken the democratic decision to approve the protocol and disapply parts of the Acts of Union where they are inconsistent with the protocol.
He described this as “modest disapplication, subject to a democratic process, mandated by parliament”.
He added that it was “entirely wrong to say there has been a breach of the Act of Union or there has been repeal of the Act of Union – much of what flows in the rest of the argument flows from that fundamental misconception”.
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On Wednesday, Mr Larkin said the protocol had “fundamentally changed” Northern Ireland’s position within the UK, in a way that was “incompatible with the guarantee” provided in the Northern Ireland Act, the legislation which implements the Good Friday Agreement.
He said that law was designed to guarantee constitutional change would not take place “without the consent of the people of Northern Ireland in a referendum”.
However Mr McGleenan said the Northern Ireland Act only dealt with the “key binary constitutional question” of whether Northern Ireland remained in the UK or became part of a united Ireland.
He said the impacts of the protocol were “a world away” from this.
In written submissions, Mr Gleenan also argued the decision to agree the Brexit withdrawal agreement and protocol was “non-justiciable”, or not suitable for a judge to decide.
“The appellants have repeatedly sought to challenge the very permissibility of the negotiations with the European Union about the terms of exit from the EU and the power to reach agreement on the terms that were agreed,” he said.
“It remains the case that any engagement between the government and the EU on the terms of exit from the EU are non-justiciable matters.
“They relate to the conduct of foreign affairs; and are in the realm of politics, and are in nature entirely unsuited to determination by a court on a judicial review application.”
Mr McGleenan added that the “exclusion of courts from political questions” was well-established.
The hearing has finished and the Court will deliver a judgement some time in 2023.
Meanwhile, the president of the European Commission has said that if “there’s the political will in the UK” the EU can reach a “positive conclusion” with it over the Northern Ireland protocol.
Ursula von der Leyen made her remarks at a joint news conference with Taoiseach (Irish Prime Minister) Micheál Martin.
Addressing both houses of the Oireachtas (Irish parliament) later on Thursday afternoon as part of a two-day visit to Ireland, Ms von der Leyen referred to “a new pragmatic spirit” in talks with London.
She said her contacts with Prime Minister Rishi Sunak were encouraging and Ireland could count on continued EU support.
She told the Oireachtas “there can be no hard border on the island of Ireland”.
President von der Leyen was making her address to mark 50 years of Irish membership of what is now the European Union.
“Brexit will not become an obstacle to the path of reconciliation,” she added.
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