The Northern Ireland Secretary of State said yesterday that it was now-or-never time for the future of the Belfast Agreement. He called on all its signatories to "knuckle down and get cracking" to implement all of its provisions.
Mr Mandelson was speaking in Omagh during engagements that included a private meeting with relatives of the 29 people killed in the 1998 bombing.
He said cross-party and cross-community confidence had to be restored in Northern Ireland if the peace process was to survive difficulties.
"Unionists have lost confidence that republicans are prepared to put their arms beyond use if they are not even prepared to re-engage with the de Chastelain commission.
"Republicans and nationalists have lost confidence in unionists that they are fully committed to the agreement and if they are prepared to make threats and are prepared to take the actions they are contemplating now.
"All I would say to both sides is that the bottom line in all of this is, as ever, that we have to work together to rebuild confidence and restore trust in everyone's intentions, because if we don't do that, if we don't work together, then we will simply fall back into the past and all that that means.
"You just have to look at what's happening in the Middle East to see the alternative to continuing with this peace process, with making the Good Friday agreement work and implementing it in every single respect."
He said he would be joining with the Irish Government in bringing the sides together for talks.
Mr Mandelson said the best way for confidence to be restored was for the signatories to the agreement to "knuckle down and get cracking" with implementing all of the agreement.
"It cannot be cherry-picked. You can't have one bit moving ahead of others, you can't have certain things which are inconvenient to some people simply being swept under the carpet.
"That is no way to make progress at all . . . That is why we have got to talk . . .
"If we lose it there will be no replacement, there will be no second chance, there will be no coming back. It is now or never for this agreement, and that's why people have really got to devote themselves to making it work", he said.