The British Prime Minister, Mr Tony Blair, is today expected to signal his belief that a full-blown crisis could threaten the Belfast Agreement unless paramilitaries complete the transition from terrorism to democracy.
Ahead of the Hillsborough summit of the pro-agreement parties - which Mr Blair will jointly chair with the Taoiseach, Mr Ahern - British sources suggested Mr Blair would want to acknowledge "an underlying crisis" of unionist confidence "which could snap" either in the autumn or in next May's Assembly elections.
London appears to accept such a crisis could take the form of a serious challenge to Mr David Trimble's leadership of the Ulster Unionist Party in the autumn, or a UUP defeat to the DUP in next year's poll.
Following talks with Mr Blair at 10 Downing Street yesterday, Mr Trimble said Mr Blair at Hillsborough had to restore confidence by demonstrating that republican paramilitaries could not maintain "a halfway house where they are ostensibly on ceasefire but actually involved in violence at street level." And he again insisted that, during today's talks, Mr Blair should "do and say things to make it clear that that objective has to take place and that there will be consequences if there is not movement in that direction."
However, there was no indication last night that Mr Blair was preparing to embrace a proposal by Mr Trimble that the British government bypass the need for a cross-community vote in the Assembly and exclude parties from office in Northern Ireland's power-sharing Executive. Whitehall sources last night again questioned the legality and political practicality of such a move.
Earlier this week, Mr Trimble warned the British and Irish governments that today's talks may be a last chance to resolve the situation. However, following yesterday's Downing Street meeting, Mr Blair's official spokesman said he did not expect the Hillsborough summit "to produce any instant initiatives." At the same time, Mr Blair - in reply to a letter from the Conservative leader, Mr Iain Duncan Smith - was resisting Conservative and Unionist pressure to bar people with terrorist convictions from serving as independent members of District Policing Partnerships and to reverse an earlier pledge to resolve the position of "OTRs", fugitives who would otherwise have benefited from early release schemes set up under the Belfast Agreement.
Mr Trimble clashed with Mr Blair later, in the House of Commons, over the terms of his reply to the Conservative leader, accusing the Prime Minister of passivity in the face of republican violence.
Mr Blair insisted he intended no passivity in his letter to Mr Duncan Smith. In it, Mr Blair told him: "As a matter of law, the Northern Ireland Secretary has to keep under close review the state of the ceasefires in the light of all the available information. The judgments are difficult ones, not least given the imperfections which have existed from the outset with each of the ceasefires."
Mr Blair said as time went by "it is understandable that public tolerance of those imperfections should diminish." And he insisted: "There needs to be clearer evidence that the transition away from paramilitarism is proceeding. A halfway house will not be sustainable."
A senior Conservative source said last night that Mr Blair's letter "offers a conciliatory tone but says nothing new and takes us nowhere."
Specifically, he said: "He gives no indication of at what point a 'halfway house' becomes unsustainable or when public tolerance diminishes to such an extent that it warrants him taking action."