The former American senator who brokered peace in Northern Ireland has warned there could be “serious trouble ahead” if border checks were reinstated because of Brexit.

George Mitchell, who worked with Tony Blair and Bertie Ahern to strike the peace deal in 1998, said he did not think a return to violence was inevitable in the region. But he said “the risk is high enough” for politicians to take action before any “regressive” forces in society resurge. He told BBC Radio 4’s Today programme that the greater danger was the “change in attitude” between communities that had been in conflict or had lived with physical or social barriers in the past.

“When there was a hard border, there was very little commerce, there was very little interaction between the people of Northern Ireland and the people of the Republic, and that led to stereotyping, to the demonisation of others, to attitudes that were based upon acts from the distant past,” he said.

“The open border has meant people travelling back and forth, a degree of social interaction, of commerce, of people working together. If you reinstate a hard border, you go back to the delays when stereotyping resumes, demonisation resumes, and people turn inward as opposed to outward, and they lose the benefits that come from open borders.”

Asked if he believed there could be serious trouble ahead, he said: “Yes, there could be serious trouble ahead. No society is immune from the regressive forces that are part of every problem.”

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Mitchell said peace was a fragile thing and that it would take years to make the changes in attitude between communities become a way of normal life.

He also raised concerns about the deadlock over resumption of power-sharing in the Northern Ireland. “What is most difficult to change in conflict societies is what is in the minds and hearts of people, that’s what’s hard, that’s what takes a long time to change, and that change is not completed in Northern Ireland,” he said.

The collapse of the devolved government 13 months ago, combined with uncertainty over the border, made the 20th anniversary of the Good Friday agreement, this year, “seem hollow”.

As chairman of the Northern Ireland peace talks, Mitchell worked to bring all sides together with the support of the premiers in Ireland and Britain, Bertie Ahern and Tony Blair.

