The American senator who brokered the peace deal which ended the Troubles has said border controls between Northern Ireland and the Republic would be a backwards step.
Senator George Mitchell said the ability to cross the frontier between north and south was important.
British Prime Minister Theresa May has promised no return to the borders of the past following Brexit, but there are concerns of the implications of a hard exit for the movement of people and goods.
Following a Brexit, the UK's only land border with an EU state would be in Northern Ireland.
Senator Mitchell (pictured) said: "I believe that the restoration of a militarised border with strict controls limiting traffic back and forth would have an adverse effect on relations within the island of Ireland.
"The ability to move back and forward across the border that has existed for the past several years has been very helpful in increasing commerce and also in reducing stereotypes on both sides. I think that that would be a step backward for that to occur."
Senator Mitchell , who was was President Bill Clinton's special envoy to Northern Ireland, told the BBC's The World This Weekend he did not want to prejudge any deal that may be done on the border, but hoped "constructive and thoughtful leaders on all sides" could reach an agreement that would "permit open access".
Senator Mitchell helped clinch the 1998 Good Friday Agreement between the Irish and British governments and the parties at Stormont which largely ended decades of conflict.
Asked if Brexit was a breach of the Agreement, Senator Mitchell said the deal "plainly contemplates the possibility of a vote under certain circumstances" that change Northern Ireland's constitutional position.
"The agreement plainly provides that the political status of Northern Ireland can be determined or changed only through a vote - and it's the informed consent through a vote - of the people of Northern Ireland," he said.
He added: "I'll leave those arguments to the people of Northern Ireland and the UK."
Senator Mitchell also said Britain and Ireland's membership of the European Union had helped create the conditions for the peace process.