By Amanda Ferguson

BELFAST (Reuters) - Northern Ireland's High Court will rule on Friday on a challenge against British plans to leave the European Union without a vote in parliament, the first judgment in legal cases that are being closely watched by politicians and markets.

A cross-party group of politicians, including members of the British province's largest Irish nationalist parties, brought the challenge earlier this month, arguing that a vote in the Northern Ireland regional assembly should also be required.

Justice Paul Maguire told the three-day hearing that he would consider the case and reconvene at an unspecified date. One of the plaintiff told Reuters on Wednesday that the judge had notified them that he was ready to give his ruling.

"The legal teams have to come down to court on Friday morning at 10am for the judgment," said Raymond McCord, the father of a man killed by loyalist paramilitaries in 1997 whose case was heard in parallel with the politicians'.

"We are confident. I believe and my legal people believe we have a very strong case."

The politicians pursuing the High Court case argue that the British government is legally obliged to maintain the statutory recognition of the 1998 Good Friday Agreement between Britain and Ireland, which contains references to the EU.

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.

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.

Lawyers for the British government told the court that it was not "illegitimate" for Britain's prime minister to begin the process of leaving the EU without parliamentary approval

Campaigners have taken similar action in London to argue Prime Minister Theresa May and her ministers do not have the authority to invoke Article 50 of the EU Lisbon Treaty, the mechanism by which a nation can leave the bloc, without the explicit backing of parliament.

One of the applicants in that challenge, Northern Irishman Fergal McFerran, told Reuters that he understood that the judgment in the London case was expected next week.

Whatever the result in either case, the losing side will almost certainly appeal to the Supreme Court, the UK's highest judicial body.

(Editing by Padraic Halpin and Toby Chopra)