Mr Jeffrey Donaldson and two fellow MPs are believed to be set to resign the Ulster Unionist whip at Westminster today, while remaining inside the UUP, in a fresh challenge to the authority of the party leader, Mr David Trimble.
This emerged last night as reliable unionist sources told The Irish Times that Mr Donaldson had concluded his consultations with colleagues over his resignation threat and had decided to continue the battle against Mr Trimble from inside the party.
Mr Donaldson, the Rev Martin Smyth and Mr David Burnside are expected to confirm their intention at an early-morning press conference in a Belfast hotel, likely to be chaired by the former UUP leader, Lord Molyneaux. Mr Trimble is scheduled to meet the Taoiseach, Mr Ahern, in Dublin later today.
The move by Mr Donaldson, Mr Smyth and Mr Burnside will be presented by them as a first step towards an eventual realignment of unionist forces and the creation of a broadly-based alliance to mobilise anti-Belfast Agreement opinion for any Assembly election.
The immediate effect will be to deny the Ulster Unionists their current status as the fourth-largest party in the House of Commons.
With just two remaining colleagues, Mr Trimble's parliamentary party would then trail behind the three nationalist parties - Sinn Féin, Plaid Cymru and the Scottish National Party - leaving it and the SDLP as the two smallest parties at Westminster.
There is no suggestion that Mr Donaldson, Mr Smyth and Mr Burnside plan to apply for the DUP whip, although the trio are expected to signal their desire for immediate talks with the DUP about how they might co-operate more closely in the Commons. With the two groups between them having 8 MPs to Mr Trimble's 3, one possibility not being ruled out is that they might agree a joint whipping arrangement at Westminster, at least in the first instance.
Beyond procedural manoeuvres at Westminster, however, the intended political message from today's press conference to be directed at the British and Irish governments and the other pro-agreement parties will be that Mr Trimble represents a minority of unionists and will be unable to negotiate any deal with Sinn Féin and the IRA likely to secure the restoration of the Stormont Assembly and Executive.
Asked if a decision to resign the party whip would not in fact represent a climbdown by Mr Donaldson, and merely formalise the existing divisions within the UUP, one well-placed source insisted: "Absolutely not. This will remove the last façade. It will show very, very clearly that David Trimble is the leader of a rump and not a party.
"And it will present a big challenge to the two governments. This isn't just about the internal affairs of unionism. This raises process issues which the two governments and the other parties will not be able to ignore."
The reaction of the DUP will be crucial to the overall assessment of the significance of today's expected developments, as well as to the assessment of Mr Donaldson's personal position and standing a week after what he termed a "defining moment" for the ruling Ulster Unionist Council ended in a narrow victory for Mr Trimble.
Last night, one DUP source suggested that a decision by Mr Donaldson and his colleagues to resign the whip while remaining inside the UUP would only be credible "if they offer a road map and a timetable for realignment".
However, he added: "Realignment, by definition, means ultimately leaving the Ulster Unionist Party."
Apart from the persuasion of his more senior colleagues, it seems clear that Mr Donaldson's calculations have also been influenced by evidence that some members of the pro-agreement camp are again questioning the viability of Mr Trimble's leadership given that he survived last week's challenge by the narrowest (54%-46%) margin yet.