Hillary Clinton has dramatically intervened in the dispute over Brexit to warn that the hand of history will be “heavy and unforgiving” if a hard border is allowed to undermine peace in Northern Ireland.

Marking the 20th anniversary of the Good Friday agreement with her first newspaper opinion piece since losing the US election, the former secretary of state becomes the most senior non-European yet to sound the alarm.

Failure to find a post-Brexit solution that avoids new frontier controls when the UK leaves the EU risks breaching the terms of a wider Irish peace deal that was struck with considerable US support.

“We cannot allow Brexit to undermine the peace that people voted, fought and even died for,” Clinton writes in the Guardian. “Reinstating the border would be an enormous setback, returning to the ‘bad old days’ when communities would once again be set apart.”

UK and EU negotiators agreed to temporarily park the vexed question before last month’s transition deal but urgently need to find a solution before the next European summit in June.

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Overseas interventions, such as that by the former US president Barack Obama during the EU referendum, have so far appeared to have little impact on domestic UK opinion, but more recently former prime ministers John Major and Tony Blair – both heavily involved in brokering talks in Northern Ireland – have warned of the dangers to Irish peace from a disorderly Brexit.

Clinton adapts a famous Blair quote from the time of the original Belfast agreement to warn: “If short-term interests take precedent over solving the long-term challenges that still exist in Northern Ireland, then it is clear that the hand of history will be both heavy and unforgiving.”

She adds: “These are difficult times for Northern Ireland, and for our world. As the Brexit debate wages on, I continue to believe in the value of the European Union, and of a Europe that is whole, free and at peace.”

Speaking in Dublin on Monday evening, Bill Clinton said that the Good Friday agreement was still a beacon of hope to other people locked in conflict around the world. The former US president said it “shows us how we might go forward together” across the planet.

He said the architects of the agreement had created “a fine piece of work” that others in ethnic wars could learn from.

He was speaking at the Clinton Institute at University College Dublin before a trip to Belfast on Tuesday.

The former US senator, George Mitchell, who successfully chaired talks leading to the Good Friday agreement, has also said that London and Dublin should put aside their disagreements over Brexit to protect the deal.



However, he added that he also believed the agreement, which turns 20 years old this week, would survive any post-Brexit turbulence.

The retired senator and diplomat said Anglo-Irish governmental cooperation had been the “critical first factor” in the process leading to the Good Friday peace deal, and that Northern Ireland now needed the same strong unity between the two governments to defend the gains of the agreement as well as restoring power sharing to the region.

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Speaking before the 20th anniversary of the agreement on Tuesday, Mitchell said: “I have said many times that the factor that led to the talks was the decision by the governments of the UK and Ireland to cooperate, to try to establish a basis for those negotiations. This was indisputably the critical first factor and but for that I don’t think there would have been any talks or any agreement.

“You can draw from that time in history that this governmental cooperation must continue.”

He said he believed the agreement could survive the dangers posed by the UK pulling out of the EU and creating a land border betweenNorthern Ireland and the Republic, but added: “It depends on what you mean by Brexit and how it is applied.”



He continued: “The government of the UK and the EU have both publicly committed that any Brexit agreement they reach will not include the restoration of a hard border.



“Now saying that and doing that are two different things. My view is that I feel that Brexit is a mistake for the UK but it was a democratically taken decision and must therefore be respected. The question now is what kind of agreement is going to be negotiated between the EU and the UK.”

He added: “A hard border is essential to avoid and I note that … the UK government and the EU, have publicly committed themselves to avoiding that. So we have to support and encourage the final outcome of Brexit where there is no hard border. Because a hard border would lead to the conditions that created demonisation and stereotyping between Britain and Ireland in the past but I do think both the British government and the EU are genuine in their wish to avoid a hard border and so I believe the agreement will survive.”

Mitchell, who also worked on peace accords in the Middle East and Bosnia, said that despite the protectionism and Trump administration’s America First policies, Washington politicians would still back US investment in Northern Ireland.

“Doing that is one of the most important things we can achieve for the people of Northern Ireland 20 years on from the Good Friday agreement,” Mitchell said.