BREXIT could potentially lead to "a resumption of violence", former senator George Mitchell has said.
The ex-US senator, who was instrumental in forging the 1998 Good Friday Agreement, said the UK's departure from the EU in March will be "harmful" to the British economy and a "disaster" for the Republic.
Writing in the Impartial Reporter, Mr Mitchell said he hoped Brexit would not spark a return to violence.
"The risk of a resumption of violence, impossible to measure with precision, could increase, although we must all hope and pray to the contrary," he said.
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Mr Mitchell said Brexit could lead to "other European countries leaving the EU" and the possible dissolution of the whole union. He said a hard Brexit, which would lead to customs posts on the border, "is in no one's interest".
"A hard border between Ireland and Northern Ireland would be harmful to both," he said.
Mr Mitchell said both economies and the overall peace process had benefitted from an open frontier.
And he said the EU and British government should stick to their promise that there will be no hard border.
"If they do not, the consequences will be severe for all," he said.
The former US politician said although the 1998 agreement remained "an historic achievement" it did not "guarantee peace, stability, or reconciliation".
He said the "real heroes" of the agreement "were the people of Northern Ireland and their political leaders".
Mr Mitchell added he hoped that today's leaders would demonstrate the same "levels of courage and vision".
Meanwhile, a conservative think-tank has claimed that the Irish government's Brexit stance contravenes the Good Friday Agreement.
In a research note for Policy Exchange, former Ulster Unionist leader David Trimble, his former special adviser Graham Gudgin and ex-Irish diplomat Ray Bassett have argued that Prime Minister Theresa May's stance is closer to the "original principles" of the agreement than the EU or Irish government.
Lord Trimble accused the Irish government of "riding roughshod" over the agreement.
And he said the EU was misrepresenting the 1998 agreement's principal of consent which says that any change to the north's constitutional status must decided by a majority of the people in Northern Ireland.
"As Policy Exchange observes, there is a genuine risk that Northern Ireland will end up as part of an effective EU protectorate, without the say-so of the Northern Ireland Assembly," he said.
"This would be an appalling breach of the principle of consent, which runs through the agreement."
He said trade talks between the UK and EU should include an 'Ireland chapter' in which "both sides genuinely try to ensure an invisible border and preserve existing cross-border cooperation".
