But his wife Heather pointed out that on the same day, 61 children were born in Northern Ireland, and he owed it to them to return to Belfast.

“When you approved the agreement, you were also talking to Israelis and Palestinians, to Colombians, to Africans, to Asians, to Americans,” he told the audience at Queen’s University Belfast.

“In fact, you were talking to the world. This is an agreement for peace and for the future, not just here, but everywhere.

“We are living in fractured times. We need you. We need your ongoing patience, stamina, and perseverance,” said Mitchell, attending his first major public event since he was diagnosed with leukaemia in 2020.

“We need people who believe, who know, that the possible does exist within the impossible. Don’t let it slip away.”

Twenty-five years since the April 1998 agreement ended the three-decade-long “Troubles” in Northern Ireland, the UK-ruled territory is locked in new political paralysis.

What is the Good Friday Agreement that helped bring peace to Northern Ireland?

The pro-UK Democratic Unionist party (DUP) has for more than a year been boycotting the power-sharing government, in protest at post-Brexit trading arrangements agreed by London and the European Union.

Mitchell was speaking alongside former US secretary of state Hillary Clinton, who said a new deal on the Brexit trading front agreed by London and Brussels, called the Windsor Framework, offered much promise.

“You stand as an example to the world of how even the staunchest adversaries can overcome differences to work together for the common and greater good,” she told the conference.

“So I encourage everyone now to move forward with the same spirit of unstoppable grit and resolve that brought the peace 25 years ago.

“Your friends in the United States will be behind you all the way, as you work toward peace, prosperity and stability that last.”

The three-day anniversary conference ends on Wednesday with speeches from Bill Clinton, UK Prime Minister Rishi Sunak, Irish premier Leo Varadkar and EU leaders Charles Michel and Ursula von der Leyen.

An emotional Biden tells Irish parliament: ‘I’m home’

It comes after US President Joe Biden last week visited Northern Ireland and the Republic of Ireland, his ancestral homeland, during which he was accused by unionists of siding with Irish nationalism.

The White House denied that, and Varadkar said after meeting Biden in Dublin that the president was “keen to be supportive in any way that he can” to uphold the peace in Northern Ireland.

Among other politicians who met Biden in Dublin was Gerry Adams, whose Sinn Fein party was once the political wing of the Irish Republican Army in Northern Ireland.

Attending the Belfast conference, Adams said of the current impasse: “I think that unionism needs to give itself a shake, generally speaking.

“Get into the institutions and then argue it out, revise, review, whatever you want, but in the first place go to where you were sent by the electorate,” he said.

Twenty-five years on, calls have been growing from moderates in Northern Ireland for an overhaul of the Good Friday Agreement so that neither the DUP nor Sinn Fein can exercise a veto over the local government at Stormont.

Former Irish premier Bertie Ahern, who helped to negotiate the 1998 pact with Britain’s Tony Blair, said reform could be discussed once Stormont is back up and running.

“The sooner the better. The one thing that concerns me is that status quos don’t work,” he said.