The Good Friday Agreement would never have happened without the European Union, a former US senator who helped broker the deal has said.
George Mitchell chaired the talks which led to the 1998 peace agreement.
Speaking to BBC Talkback, he said Brexit was a mistake but the vote to leave the EU must be respected.
He also said he hoped power sharing can be restored in Northern Ireland but he knew at the time of the agreement that there would be future issues to solve.
However, he also said that the "serious" issues faced by politicians are not as difficult as those faced by political leaders in 1998.
"I hope and pray that the current leaders of Northern Ireland - of all parties - have the same courage, determination, vision and commitment to the people of Northern Ireland to resolve their differences," he said.
The Northern Ireland executive originally established by the Good Friday Agreement collapsed in January, and the last round of power-sharing talks ended in stalemate in June.
Mr Mitchell said the question around cultural and identity issues should be how can they be put to a positive use.
"People must recognise that while they have priorities, they have to get their priorities in order. And the highest priority is to the people as a whole," he said.
Mr Mitchell said the European Union played a part in thawing relations between the Republic of Ireland and the United Kingdom which enabled the Northern Ireland peace process and was central to the Good Friday Agreement.
"I don't think the European Union was essential in the [Good Friday Agreement] talks themselves, but I believe the talks would never have occurred had there not been a European Union."
He said Brexit was a "democratically taken decision which therefore must be respected" but felt that "it will historically prove to be a major error on the part of the people of the UK".
The consequences of Brexit on the island of Ireland could prove to be more intense and negative than in England, Scotland and Wales, he added.
You can listen to the full interview with George Mitchell on BBC Talkback at 12:00 GMT on BBC Radio Ulster.