ARTICLE AD BOX
The prime minister has urged unionist politicians at Stormont to get power-sharing "up and running again".
Rishi Sunak gave the closing speech at a three-day event at Queen's University Belfast marking the agreement's 25th anniversary.
He called it a "profound concern" that power-sharing had not been in place for nine of those years.
Democratic Unionist Party (DUP) leader Sir Jeffrey Donaldson said the conference had a lack of realism.
His party has blocked the formation of an executive and the functioning of the assembly in protest against post-Brexit trading arrangements for Northern Ireland.
Mr Sunak said the collapse of the institutions "will not bolster the cause of unionism" in the long-term. "I believe that deeply," he added.
"When we look back and 25 years from now, surely we should aspire for our legacy to be nothing less than this: that the institutions have been up and running every single year," the prime minister said.
He hailed the agreement as the "extraordinary political achievement of our lifetimes".
Sir Jeffrey said: "There is a realism in London that frankly there isn't at the event at Queens.
"A realism that we need to sort this out, that whilst we will have lots of people that will tell is what the problem is, I haven't heard anyone come forward with a solution."
Later, the prime minister will meet Stormont's political leaders and host a gala dinner at Hillsborough Castle.
Other current and former world leaders also addressed the conference on Wednesday.
Among them were former US President Bill Clinton and Taoiseach (Irish prime minister) Leo Varadkar.
'Get the show on the road'
In his speech, Mr Clinton urged local politicians to get the Stormont "show on the road".
"The biggest roadblock that Brexit posed for Northern Ireland's political and economic future has been dramatically mitigated," he told the audience.
"This whole deal was never supposed to be an engine of obstruction. The agreement was never supposed to be used to make sure there could be no self-government."
Mr Varadkar reiterated the Irish government's agreement to work with the British government, as well as Stormont politicians, to make the process work across the islands.
He added that Northern Ireland works best when the Irish and British governments work with an agreed strategy.
'NI is a cool place'
Presidents of the European Council and European Commission, Charles Michel and Ursula von der Leyen, also addressed the final day of the conference.
Ms von der Leyen praised the people of Northern Ireland for being the masters of their own destiny.
She told the conference it was now known as a "cool place" for Europeans to visit.
"This is only possible because the leaders and people decided to plant a seed in soil previously stained with blood and tears," she added.
Ursula von der Leyen's presence in Belfast reminds us of an uncomfortable truth.
The Windsor Framework signed with Prime Minister Rishi Sunak has so far failed in its main aim, restoring devolution at Stormont.
She called it "a new beginning for old friends".
She referred to the role of former DUP leader Ian Paisley and his friendship with Martin McGuinness as "one of the miracles" of the Good Friday Agreement.
"History now calls on today's leaders to embark on a similar path."
It was as close as she came to mentioning the great big elephant in the room.
The dinner on Wednesday night will be the culmination of events marking the anniversary.
It is understood a number of former prime ministers will be among those attending.
Also on the guestlist are former US President Bill Clinton and his wife, former US Secretary of State Hillary Clinton, ex-US Senator George Mitchell - who chaired the peace talks - as well as former taoiseach (Irish prime minister) Bertie Ahern.
Representatives of Northern Ireland's political parties, the Irish government, as well as EU and community representatives will also be there.
The events follow US President Joe Biden's visit to Northern Ireland and the Republic of Ireland last week.
This week the Clintons, who have been long-time supporters of the Northern Ireland peace process, have been visiting Northern Ireland, paying tribute to those who helped make the agreement possible.
On Tuesday evening, Bill Clinton told an event at the Guildhall in Londonderry that the leaders behind the agreement, David Trimble and John Hume, "put their lives and careers on the line" for peace.
What is the Good Friday Agreement?
The Good Friday or Belfast Agreement was a political deal designed to bring an end to 30 years of violent conflict known as the Troubles.
Signed on 10 April 1998 and approved by public votes in Northern Ireland and the Republic of Ireland, it is based on the idea of co-operation between communities.
It helped to set up a new government for Northern Ireland, representing both nationalists and unionists.