He also confirmed that he would like John Major - who laid the long-term foundations for the deal - to play a role in the referendum campaign.
On cue, Mr Major laid down two reassuring lines for the Unionists. He told GMTV that Sinn Fein would have to make "progress" on the decommissioning of weapons, saying: "Clearly, you cannot go into a democratic assembly with an Armalite under the table." But he added his belief that the deal marked an end of Sinn Fein ambitions for a united Ireland.
"I think there is a realisation," Mr Major said, "that that objective is not going to be readily obtained, and probably isn't going to be obtained at all, and they will have to settle for what is available, and I think they have made that judgement."
Mr Blair, meanwhile, played up the historic opportunity available. "This is a chance for people to make a new start and this is something all of the world is willing them to do," he said.
But the Prime Minister warned that there would be people who would not want to make the necessary changes, who would do all they could to stop the agreement, and there was a lot of work to be done if the deal was to really deliver peace on the ground.
"It is very much a beginning," he said. "What we have done is we have got in place a plan for the architecture of peace, but the building has still got to be built and that's done by the people, the parties on board.
"We shouldn't take anything for granted or think other than this is going to be a difficult process over this next period of time. It is going to be.
"What we have done so far has primed people, and if people exercise vision and imagination then it can be done. There is no doubt about that because the principles of this are right."
The Prime Minister added that there was much respect for Mr Major in Northern Ireland: "I hope very much that he could go to Northern Ireland and play a role in the referendum campaign because he was the person that started the whole process off."
He also paid special tribute to the courage of David Trimble, the Ulster Unionist leader, for taking on the opponents within his own party ranks. "He has had the vision and the political courage to say that `this is something that we can manage now for the future', because for the first time in 70 years, the principle of consent is now accepted," Mr Blair said.Reuse content