Mr Tony Blair has said only the Northern Ireland parties, not the British and Irish governments, can break the impasse in the peace process.
The prime minister, in his first public comment since last week's talks at Stormont ended in disagreement and deepening discord, commended the parties for their efforts.
He praised republicans in particular for their commitment to what he called an "enduring peace".
But he said the governments could only help the parties reach agreement; they could not do it for them.
Speaking to the BBC's David Frost on the morning of the Labour Party conference, Mr Blair said: "I think the republicans have come an enormous way and are really doing their best to try and bring about an enduring peace."
In contradiction to the optimism of Northern Ireland Office sources last week, when it was hoped that agreement could be secured in the short term, Mr Blair said: "I hope we can make sure that over the coming weeks, the republicans, nationalists and unionists manage to make a lasting agreement. I think the prospects of this are reasonable but in the end, it's only they who can do it - we can help but we can't do it."
His comments follow demands from Sinn Féin's Mr Martin McGuinness on Friday for the British government "to defend the agreement" from the DUP anti-agreement strategy.
It is anticipated that the two governments will agree a common position on proposals to be put to the parties in another effort to find agreement on contentious issues of ministerial accountability, the joint nature of the office of the First and Deputy First Minister, and the Executive's collective nature.
Sinn Féin and the SDLP have criticised the DUP over its insistence that these issues be radically addressed, accusing the largest unionist party of attempting to destroy the Belfast Agreement and damage the all-Ireland elements of the 1998 accord.
The atmosphere between the two sides worsened yesterday with further sharp exchanges.
The SDLP leader, Mr Mark Durkan, addressing a specially convened party meeting following last week's Stormont talks, said: "The SDLP position is very simple and very clear: we will defend and protect the fundamentals of the Good Friday Agreement, alone if necessary. We are not opposed to improvements consistent with the agreement, but that is not what the DUP is seeking. They want to hollow out the agreement."
He identified three fundamentals of the accord. "First, the office of the First Minister and Deputy First Minister is a single office representing the coming together of our two communities. They must be elected together by all because they must act and work together for all every day. Second, ministers must not be subject to a running DUP veto. Third, we want to develop and expand the all-Ireland institutions, not to have them frozen in time."
He said the DUP was "abusing its mandate" and accused the party of blocking progress until they get the agreement changed. "Who is to say they would not do this week in and week out in the Assembly and Executive?" he asked.
However, Ms Arlene Foster, a Fermanagh Assembly member who defected to the DUP along with Mr Jeffrey Donaldson last January, countered: "My colleagues and I have spent hours in meetings this past week and some of the most depressing have been facing the complete intransigence of the SDLP."
The DUP was "frustrated and bewildered" by the SDLP's "total unwillingness to make any movement," she said.