Direct rule in Northern Ireland or a hard border after Brexit could lead to “serious trouble” and a return to the violence of the 1970s and 1980s, one of the key brokers of the Good Friday Agreement has warned.

George Mitchell, who as United States Special Envoy for Northern Ireland chaired the talks that led to the historic agreement, said a hardening of the border could mean “stereotyping resumes, demonisation resumes and people turn inward”.

He also warned that failure to restore power-sharing at the Stormont assembly could lead “regressive forces” to reignite their campaign of violence.

The British and Irish governments and the EU have all ruled out a hard border in Northern Ireland after Brexit, but European leaders have said the only way to guarantee this is through some form of customs union – something the UK has repeatedly ruled out.

Amid fears that border checks could be reintroduced, Theresa May used a major speech in London on Friday to outline how the Government believes an open border could be maintained.

Mr Mitchell said doing so is critical to maintaining peace in Northern Ireland.

“The real danger of a hard border is not the immediate resurgence of violence – although that certainly is a problem – it is the change in attitude,” he told BBC Radio 4’s Today programme.

“When there was a hard border, there was very little commerce, very little travel, very little interaction between the people of Northern Ireland and the people of the Republic.

“That led to stereotyping, to the demonisation of others, to attitudes that were based upon acts from the distant past.”

He added: “The open border has meant people travelling back and forth, a degree of social intercourse, of commerce, of people working together. Stereotypes have not gone completely but they have been dramatically reduced.

“The real danger is if you reinstate a hard border, you go back to the days where stereotyping resumes, demonisation resumes and people turn inward as opposed to outwards and they lose the benefits that come from open borders, open societies and trade.”

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Mr Mitchell also warned about the dangers of direct rule from Westminster being reintroduced in place of devolution in Northern Ireland.

The region has been without a functioning executive since power-sharing at the Stormont assembly collapsed in January 2017.

With months of talks between the parties having failed to reach an agreement, there have been calls for the UK Government to restore direct rule in order to pass a budget and take key decisions.

Theresa May: As Prime Minister of the whole of the United Kingdom, I won't let Brexit affect Northern Ireland progress

Mr Mitchell said militants could exploit the tensions to spark further violence.

“There could be serious trouble ahead,” he said. “No society is immune from the regressive forces that are part of every problem.

“While the overwhelming majority of people in Northern Ireland don’t want to return to the violence of The Troubles, there remain very small but dangerous groups who don’t feel that there should have been any compromise at all, who are still willing to use violence to try to resolve their political differences.

“When violence gets going it tends to drive people back into their tribal positions, and so I don’t think anyone can say with absolute confidence that there will never be a return to violence. We have to keep that in mind at all times.”

Mr Mitchell will be travelling from the US to celebrate the 20th anniversary of the Good Friday Agreement next month but said the celebrations felt “hollow” given the current state of politics in Northern Ireland.

He said that, while the problems now are not comparable to The Troubles, the current situation is “very serious and it could become worse”.