The United Kingdom's departure from the European Union could have a "catastrophic" impact on the Northern Ireland peace process, the High Court heard today.
Lawyers for the father of a loyalist paramilitary murder victim predicted Brexit would cause constitutional upheaval amid renewed calls for a united Ireland.
With Northern Ireland having voted to remain in the EU, a judge was also told the Good Friday Agreement has given its public sole sovereignty on the issue.
Ronan Lavery QC said: "A change so profound as withdrawing Northern Ireland from the European Union requires the consent of the people of Northen Ireland."
His client, victims campaigner Raymond McCord is mounting a landmark legal challenge to Brexit.
Politicians including Alliance MLA David Ford, SDLP leader Colum Eastwood, Sinn Fein Assemblyman John O'Dowd and Steven Agnew of the Green Party are also seeking to judicially review the British Government's move towards quitting the EU.
Prime Minister Theresa May has announced she will trigger Article 50 of the Lisbon Treaty, the formal process for confirming the UK's departure, by the end of March 2017.
But proceedings underway in Belfast claim the move is unlawful without first securing Parliamentary authorisation.
Even though the June 23 referendum backed Brexit, a 56% majority of voters in Northern Ireland wanted to remain.
Mr McCord, whose son Raymond McCord Jr was killed by the UVF in north Belfast in 1997, believes they have a legal right to resist being forced out.
His barrister argued that the 1998 Good Friday Agreement led to a transfer in sovereignty over Northern Ireland's constitutional position.
"Only the people of Northern Ireland can decide whether there will or will not be a change in the constitutional status," Mr Lavery argued.
Mr Justice Maguire was told Brexit would have the greatest impact on the only UK region which shares a land border with the rest of the EU.
The Northern Irish situation was described as a microcosm of the European project to render 19th Century notions of nationalism irrelevant and lead to borders disappearing.
Concerns were also raised that a departure from the EU could lead to a repeal of the Human Rights Act and possibly even withdrawal from the European Convention on Human Rights.
Mr McCord, who refers to himself as British and a unionist, is now considering applying for an Irish passport, the court heard.
His lawyer raised alternative potential consequences for those on either side of the political divide.
Nationalists seeking a united Ireland may have their aspirations impeded by the creation of a "hard border", it was claimed.
But Mr Lavery went on to suggested: "What this upheaval has also caused is the nationalists call for a border poll and a very real threat to the Union as it exists."
He questioned whether Northern Ireland would be incapable of "succeeding" as an entity and be forced into a united Ireland.
"One doesn't know what way history will turn, but one thing is very real: withdrawal from the European Union could have a catastrophic effect on the peace process and the delicate constitutional balance we have reached."
The hearing continues.