In a performance primarily intended to encourage doubters in David Trimble's Ulster Unionist party to overcome their misgivings and back the new deal, Mr Mandelson declared he believed "decommissioning will happen as a natural and essential development of the peace process".
His words were apparently not enough to persuade Mr Trimble's deputy John Taylor to switch sides and support the deal when it goes before the Ulster Council for endorsement on Saturday. Nor were the words welcome to Sinn Fein, whose chief negotiator Martin McGuinness said "there is no default mechanism in the Agreement".
Mr Mandelson's message was clear. He said: "It would pain me to do so, but I would not shrink from suspending the institutions should it prove necessary, thus restoring the status quo so as to consider how to rectify the default."
There was debate yesterday whether the technicalities of the Agreement allowed Mr Mandelson to give that pledge. But he made equally clear that the British and Irish governments were at one in saying if there was default on de-commissioning, they "will take the steps necessary to cease immediately the operation of the institutions".
Unionists said after the Commons exchanges that they would continue to seek further assurances from the Government. Mr Mandelson set out a brisk timetable, with ministers in the new executive to be nominated on Monday if the Ulster Unionist Council gives the go-ahead. On Tuesday, Parliament will be asked to approve the devolution of powers to Belfast, with devolution actually "going live" on Thursday.
Mr Taylor said many Unionists were hesistant because they did not know what would happen in the event of failure. He added: "When they have assurances on that they might go with this, but until then it is in real trouble."
This assessment goes against that of Trimble supporters, who have begun to predict he will win a large majority on Saturday. His weakness is seen as the absence of an absolute guarantee that IRA decommissioning will happen, but the failure of his critics to produce a coherent alternative strategy appears to be outweighing this.
John Major said Mr Mandelson's approach represented a justified gamble. He believed a settlement was within reach but warned: "Difficulties may lie ahead with violence from fringe groups and we need to prepare ourselves for that possibility."
Mr Trimble told the House the basis existed to proceed on both devolution and decommissioning, but said one depended on the other. He added: "Without de-commissioning, devolution will not survive, if only because our position will become untenable. We would not wish to remain if that were the case."
In Belfast, inter-Unionists spats continued, with the Rev Ian Paisley calling on Ulster Unionists to throw out the plan, and, with it, Mr Trimble. Ulster Unionists cited what they called Paisleyite hypocrisy, pointing out that Mr Paisley has confirmed his party will take up its ministerial seats.Reuse content