By Amanda Ferguson
BELFAST (Reuters) - Northern Ireland's High Court began hearing a legal challenge on Tuesday against British plans to leave the European Union without a vote in the Westminster parliament.
The case is being brought by a cross-party group of politicians, including members of the province's two largest Irish nationalist parties, who argue a vote in the Northern Ireland regional assembly should also be required.
While overall, 52 percent of the United Kingdom voted in favor of leaving the EU in June's referendum, a majority - 56 percent - of those voting in Northern Ireland backed remaining in the bloc.
Irish nationalist parties oppose Brexit, saying it could undermine a 1998 peace deal, reinstate a hard border with the Republic of Ireland and cut EU cross-community funding.
The province's largest party, the pro-British Democratic Unionist Party, supports Brexit and the vote has heightened tensions between its leader, First Minister Arlene Foster, and her Irish nationalist coalition partners.
The British government has said its legal advice is that the prime minister has the right to initiate the formal process to start a two-year countdown to an exit and does not require parliamentary approval.
Prime Minister Theresa May said on Sunday she would begin the divorce process by triggering Article 50 of the EU Lisbon Treaty by the end of March.
"A change so profound as withdrawing from the EU requires consent from the people of Northern Ireland," lawyer Ronan Lavery told the court on Tuesday. The hearing was scheduled to last two days, but the judge did not indicate when a ruling was due.
Similar legal challenges have been launched in England and London's High Court is due to hear them later this month. It is expected their outcome will be appealed to the Supreme Court, the United Kingdom's highest judicial body, to deliver the final verdict on the constitutional question in December.
The politicians taking the case in Northern Ireland argue that the British government is legally obliged to maintain the statutory recognition of the 1998 Good Friday Agreement between the United Kingdom and Ireland, which contains references to the European Union.
The agreement ended three decades of tit-for-tat killings between Catholic Irish nationalists, who want the province to unite with Ireland, and Protestant unionists, who want to remain part of the United Kingdom. The conflict left 3,600 dead.
Brexit could have a "catastrophic effect" on the peace process, Lavery told the court.
The politicians will also argue that the British government is legally obliged to safeguard EU laws incorporated into Northern Ireland law.
The court at the same time is hearing a similar case being brought by Raymond McCord, the father of a man killed by loyalist paramilitaries in 1997, who says Brexit would undermine the Good Friday Agreement, the peace process, and human rights.
(Reporting by Amanda Ferguson; Writing by Conor Humphries; Editing by Stephen Addison and Alison Williams)