Withdrawing from the EU without the consent of the people of the Northern Ireland would be unconstitutional and contrary to the Belfast Agreement, Britain’s supreme court has been told. Ronan Lavery QC told the court that successive agreements from the 1993 Downing Street Declaration onwards had transferred sovereignty from Westminster to the people of Northern Ireland.

Mr Lavery was representing Raymond McCord, a campaigner on behalf of victims of violence in the North, who claims that the consent of the Northern Ireland public is required before the British government starts the process of leaving the EU.

The supreme court is hearing the case along with the British government’s appeal against a high court ruling that MPs at Westminster must have a vote before Theresa May triggers article 50 of the Lisbon Treaty.

“We say that as a matter of the constitution of the United Kingdom, that it would be unconstitutional to withdraw from the EU without the consent of the people of Northern Ireland and we say that for two reasons. First of all, that being part of the EU is part of the constitutional settlement,” Mr Lavery said.

“But we say secondly that there has been a transfer of sovereignty by virtue of the Good Friday Agreement [Belfast Agreement], the Downing Street Declaration and section one of the Northern Ireland Act, so that in fact the people of Northern Ireland now have sovereignty over any kind of constitutional change rather than parliament.”

EU role

Minister for Foreign Affairs Charlie Flanagan, who was in London for meetings about Brexit with MPs, said the Government was not a party to the supreme court case, but he emphasised the role of the EU in the Belfast Agreement.

“The court will decide what the court decides, but the Good Friday Agreement was brought about following a very constructive and positive engagement on the part of the European Union.

“Over the past 25 years, looking at Europe’s gains and what Europe has done, I would put the Good Friday Agreement high up that ladder in terms of an EU positive. So the Good Friday Agreement is central. Obviously, that agreement was predicated upon both the UK government and the Irish Government being active and constructive members of the European Union,” he said.

Mr Flanagan added that, now that Britain had decided to leave the EU, he was determined that “the letter, the spirit and the gains consequent on the Good Friday Agreement” should be protected.