Brexit could end up going down as one of the biggest and most needless self-inflicted wounds in history, Hillary Clinton told an audience in Belfast on Wednesday.

Speaking at Queen’s University, the 2016 Democratic presidential candidate said to loud applause: “I make no excuse for being against Brexit from the start. I thought it was a bad idea before the referendum and I think it is an even worse idea now. It may well go down as one of greatest and most unnecessary self-inflicted wounds in modern history.”

She added: “Brexit seems to be going ahead one way or the other. It is crucial that however it comes out, Brexit should not be allowed to undermine the peace and prosperity that has been so dearly won here.” Clinton urged the region’s two main communities to put aside their differences to form a temporary executive to provide a unified voice on Brexit. The Northern Ireland assembly has not sat since it collapsed in January last year.

Clinton and her husband, Bill, then the US president, were integral to persuading Northern Ireland to make the compromises that led to the Good Friday Agreement.

Clinton, who was at the university to receive an honorary degree, warned that “the uncertainty surrounding Brexit for everyone is substantial, but here in Northern Ireland is acute”, saying the country was both “a contributor to the impasse and compelling reason to resolve it”.

Clinton said it was sobering that polls showed 87% of Leave voters in Northern Ireland regarded the end of the peace process as a price worth paying for Brexit. “Twenty quiet years can make it easy to take for granted that peace and prosperity will always continue. History teaches a different lesson. Peace is fragile, progress can be fleeting, and prosperity can leave.”

She accepted that the politics of coalition-building were tough, but said the current stakes went beyond politics, stressing that a functioning Northern Ireland executive weighing in on the final terms of the negotiations could have an impact.

She added that it would also send a clear message to businesses already skittish about Brexit that Northern Ireland remained open for business.

Addressing the current impasse over the post-Brexit border, Clinton said the best sustainable solution lay in protecting the Good Friday Agreement and working with an interim executive to preserve its benefits.