Former US Senator George Mitchell, one of the architects of the Good Friday Agreement, has warned reinstated border checks would harm the peace process.
Speaking to BBC Radio 4's Today programme, Mr Mitchell said that although a return to violence was not inevitable, the risk of it happening was "high enough".
Mr Mitchell said the greater risk with the reinstatement of a border would be a "change in attitude".
"If you reinstate a hard border you go back to the days when stereotyping resumes, demilitarisation resumes and people turn inward as opposed to outward and they lose the benefits that come from open borders," he said.
His comments come after Prime Minister Theresa May gave a keynote speech in London on Friday, in which she called for "pragmatic common sense" to deliver a Brexit deal.
She reiterated her commitment to avoiding a hard border between Northern Ireland and the Republic, but also emphasised a border in the Irish sea breaking up the UK's common market would be unacceptable.
Last year, then Secretary of State for Northern Ireland James Brokenshire met with George Mitchell in New York, with some speculation at the time he may have been approaching Mr Mitchell to take on a role in the negotiations to restore Northern Ireland's power-sharing Executive.
Mr Mitchell is set to travel to Belfast next month for an event to mark 20 years since the signing of the Good Friday Agreement.
Mr Mitchell chaired the talks, and will be joined by former First and Deputy First Ministers Lord David Trimble and Seamus Mallon along with former-Taoiseach Bertie Ahern.
Also attending will be the former DUP and Sinn Fein leaders Peter Robinson and Gerry Adams.