Mr John Taylor, the Ulster Unionist Party's deputy leader, yesterday withdrew his support for the Northern Ireland Bill implementing the Belfast Agreement as it cleared the House of Commons.
The Strangford MP's partial defection - he decided to abstain rather than vote against the Bill - will fuel anxieties in London and Dublin about the direction of the internal unionist debate, and the ability of the Ulster Unionist leadership to meet its commitments under the terms of the agreement.
A senior party source last night insisted that Mr Taylor's "maverick" action should not be seen as a blow to Mr David Trimble, the party leader and Northern Ireland's First Minister.
But Mr Taylor again held out the threat that he might yet vote against the provisions of the Bill when it returns to the Commons from the House of Lords in the autumn. And, in any event, the effect of his abstention yesterday was that the Northern Ireland Bill received its third reading without a single Ulster Unionist MP voting for it.
In fact, only one Northern Ireland MP, the SDLP's Mr Eddie McGrady, was in the division lobby yesterday supporting the Bill as it cleared its Commons stages ahead of the summer recess. MPs approved the Bill by 215 votes to eight, a government majority of 207.
Four Ulster Unionist MPs - Mr Jeffrey Donaldson, Mr Clifford Forsythe, Mr William Ross and Mr William Thompson - again voted against the Bill, with their colleagues, Mr Roy Beggs and the Rev Martin Smyth, acting as tellers for the Nos. They were joined in the division lobby by the Rev Ian Paisley and Mr Peter Robinson of the DUP, and two Conservatives, Mr Andrew Hunter and Mr Desmond Swayne.
Mr Trimble and Mr Seamus Mallon, the Deputy First Minister - they both voted for the Bill at its second reading on July 20th - were absent yesterday. Mr Trimble signalled earlier in the week that he would not be interrupting a family holiday in England, and Mr Mallon had a personal engagement in the North.
The UKUP leader, Mr Robert McCartney, who voted against the Bill last week, is on holiday in France.
Analysis of the voting lists shows three Northern Ireland MPs, Mr John Hume, the SDLP leader; and the Ulster Unionists Mr Ken Maginnis and Mr Cecil Walker, to have been absent for both key votes on the Bill. Mr Maginnis is recovering from major surgery on his spine and was unable to travel to London.
While the passage of the Bill was never in doubt, some pro-agreement sources inside the UUP were privately critical of Mr Trimble's absence yesterday, and his perceived willingness to vacate the parliamentary ground to his dissident colleagues.
Against that, some Westminster observers were inclined to be sympathetic given the embarrassment he would have faced voting again for the Bill with seven out of a total of nine colleagues refusing to back him.
Mr Taylor had signalled the possibility of a U-turn on the Bill even as he voted for "the principle" of it at second reading. While speculation built throughout the week about his intentions, anti-agreement unionists had predicted that he would not be present for yesterday's vote.
In the event, Mr Taylor was not, having apparently boarded a flight for northern Cyprus before the Commons proceedings resumed. His office issued a statement making it clear that his absence was deliberate and betokened at least a temporary withdrawal of support for the Bill.
In his statement, Mr Taylor said: "I voted for the Northern Ireland Bill at the end of its second reading last week. That was because I support the Belfast Agreement - the principle of which is embodied in this Bill. However, the government rejected all attempts by Conservative and Unionist MPs to amend the Bill to ensure that persons belonging to organisations which are still involved in paramilitary violence could not be members of the executive of the new Assembly.
"As FAIT (Families Against Intimidation and Terror) said, the Bill allows one wing of the republican movement to continue to kill people whilst another wing, claiming to oppose violence, sits in the government of Northern Ireland."
The statement continued:
"These double standards are unacceptable to me so I will not support the Bill today at third reading. I await the Lords' amendments in October in the hope that they will address these double standards."
The Prime Minister, Mr Tony Blair, and the Northern Secretary, Dr Mo Mowlam, have made it clear that they cannot add any preconditions to the terms of the agreement, concluded on Good Friday. If Mr Taylor maintains his present position, he can be expected to vote against the Bill in the autumn, so dramatically underlining the predicament of Mr Trimble, torn between the demands of a large section of his party and of the co-signatories to the agreement.
At the very least, Mr Taylor's manoeuvres will have strengthened Mr Trimble's already declared intention to resist Irish Government pressure for the creation of the executive in "shadow" form by the end of September.
During yesterday's debate Mr Peter Robinson, the DUP deputy leader, told Dr Mowlam: "Instead of crowing at the dispatch box, you and your ministers should hang your heads in shame."
Describing the Bill as "a monument to the political immorality and deception of this government", Mr Robinson accused Mr Blair and Mr Trimble of breaking their pledges that there would be substantial decommissioning of paramilitary weapons before prisoners were released or Sinn Fein took up seats in the Assembly.
"This Bill shows very clearly that terrorists can continue with their violence, continue killing, continue the so-called punishment beatings and still benefit from the agreement."
During a speech openly critical of his party leader, the UUP's Mr William Ross declared: "The United Kingdom will never be the same again, frankly, because it will not exist . . . that is the inevitable consequence of this Bill."