A Northern Irish court on Tuesday began hearing a legal challenge to the British government's right to begin Brexit negotiations, as the Irish government announced a cross-border forum on how to respond to Brexit.

Lawyers representing a group of cross-party politicians and activists in Northern Ireland said the change that would be started by Britain triggering Article 50 of the Lisbon Treaty - which sets the rules for a two-year negotiating process for a nation leaving the EU - is so profound that parliamentary consent is needed.

They told the High Court in Belfast that Brexit could jeopardize the 1998 Good Friday Agreement, which formally ended decades of sectarian conflict in Northern Ireland, and argued that the region's people should have the right to veto any constitutional change.

In a related move, the Irish cabinet on Tuesday agreed to open a cross-border forum on November 2 to "seek broad-based views on all-island implications" of Brexit.

The Irish government said it will host "a series of roundtable discussions with interested groups to allow for detailed consideration of Brexit issues arising on a sectoral level."

"Ireland faces unique challenges from Brexit, not least given the all-island issues that arise," Prime Minister Enda Kenny said in a statement following the cabinet meeting.

He said a summit of the North-South Ministerial Council on November 18 would be "hugely significant" for considering "how we can best protect the peace process and North-South interests in upcoming negotiations."

The cabinet discussed preparations for Brexit, including key concerns for Ireland in the economy and trade, the peace process in Northern Ireland, and the common travel area, Kenny said.

The common travel area allows Irish and British citizens to travel between the two countries without a passport and grants mutual voting rights in the two countries.

Many politicians have raised concerns that Brexit could cause a shift towards a "hard" border with passport and customs controls.

"The 'common travel area,' the nature of the border, movement of citizens and the nature of cross-border trade are now up in the air," Irish nationalist party Sinn Fein said in a statement on Tuesday.

Fifty-six per cent of voters in Northern Ireland chose to remain in the EU in a referendum on June 23, compared with 48 per cent across the United Kingdom.

Prime Minister Theresa May announced on Sunday that she plans to trigger Article 50 by the end of March.

Courts in England are expected to open hearings of similar legal challenges to Brexit in the next few weeks.

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