Brexit is the most "dangerous thing for Northern Ireland since partition", the leader of the Irish nationalist SDLP has warned.
Colum Eastwood told a UTV debate that any attempts to introduce a hard border between Northern Ireland and the Republic would be "economically and politically damaging".
He said: "We have a lot of work to do to defend Northern Ireland, to defend our citizens against the ravages of what is vast becoming a hard Tory Brexit."
Sinn Fein leader Michelle O'Neill also described Brexit as being "absolutely catastrophic for the island of Ireland".
She said: "It is going to undermine the Good Friday Agreement. It is going to be the major conversation over the next number of years.
"The British government are not going to dictate their negotiations on what happens post-Brexit. The Irish government needs to step up to the plate."
Concern over the impact of Brexit in Northern Ireland has intensified after Irish Finance Minister Michael Noonan admitted that customs officials in the Republic are looking at "contingencies".
He said the Government does not envisage having customs posts on the border, and neither does London - but warned that it depends on the Brexit negotiations.
Mr Noonan said: "Even though it is the intent of the two jurisdictions involved we still have to get it across the line in Europe and that might be five or six years down the line ... Customs are looking at contingency in the event of where the negotiations may land."
He admitted the contingency plans amid reports that Government officials were scoping out locations for the return of full "red and green channel" customs checkpoints.
They included crossings in Louth, Monaghan, Cavan, Leitrim, and Donegal, with particular focus on the M1.
The Taoiseach's position is that there will be no hard border.
Enda Kenny said recently: "I am confident that the European Union will not bring us back to a border of division."
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