The Northern Secretary has signalled British support for ending demands for short-term decommissioning, on the eve of a key meeting in Lisbon later today between Mr Ahern and Mr Blair to discuss the political deadlock.
In a speech in Dublin last night, Mr Peter Mandelson did not mention decommissioning, stressing instead the importance of ensuring that guns remain silent and that there is no threat of further violence.
His language was close to that used by the Taoiseach since his party's ardfheis over a fortnight ago, and British sources have confirmed it reflects a shift in the British position.
His speech follows Mr David Trimble's statement in Washington last week that he would consider re-entering the suspended northern executive without "guns up front". According to wellplaced sources, these two indications of flexibility will put pressure on the republican movement to show a willingness to negotiate.
However, Mr Mandelson also gave a sharp response to Irish Government demands for a more rapid reduction in the British security presence in the North. Such a reduction would come "only on an assessment that the threat has genuinely receded".
He added: "I am sure this will be clearly understood between the two governments."
The Taoiseach and British Prime Minister will meet tonight in Lisbon in an attempt to agree a joint strategy on how to restore the suspended political institutions. Mr Blair and Mr Ahern will be in Lisbon for a two-day EU summit concentrating mainly on employment, which begins tomorrow.
There is speculation that a further series of bilateral meetings between the political parties may begin next week, with talks intensifying in the run-up to Easter if any progress is being made.
Sources in both governments acknowledge that no clear strategy can be finalised until after the weekend Ulster Unionist Council meeting, at which Mr David Trimble may face at least one challenge to his authority.
In his speech last night, Mr Mandelson moved beyond comments he has made recently, suggesting some flexibility on how to approach the Northern deadlock.
As late as last Thursday, he said in Washington that "decommissioning will not go away. It is an essential part of the peace process." The issue would have to be tackled "head on", he said.
However, last night's speech pointedly made no reference to decommissioning, with the Northern Secretary instead posing two questions to which, he said, answers must be found.
"How can we all be sure, now that the guns are silent, that they will stay silent and that any threat of a return to war has gone for ever?" he asked.
"And how can we maintain the political progress made under the Good Friday agreement so that all parties feel it is being implemented in ways that are consistent with the principle of consent?"
The process of finding answers to these questions would not be easy, he said. Signalling a move away from the narrow demands for decommissioning, he said: "It cannot be reduced to the simple proposition: which comes first, the guns or the government?"
Instead of this proposition, he suggested that what was required was some form of assurance that there would be no return to violence and that weapons would no longer be used.
He said there "must be the sure knowledge - unambiguously stated - that violence will never again play a part in Northern Ireland politics".
He added: "We must find a way to eliminate the fear that those silenced weapons will ever again fire in anger, and to maintain our security and our protection in the meantime."