Lord Molyneaux, the former Ulster Unionist leader, is believed to be preparing to recommend a No vote in next week's referendum on the Belfast Agreement.
The intervention of his predecessor could prove a significant setback for Mr David Trimble, amid growing anxieties about the ineffectiveness of the unionist Yes campaign and private polling evidence that officially declared "undecided" unionist voters have been moving to the No camp at a rate of three to one.
In a further blow to Mr Trimble, it emerged last night that the Lagan Valley MP, Mr Jeffrey Donaldson, will now almost certainly seek the party's sanction to lead his constituency's candidates in next month's elections for the Northern Ireland assembly. This would represent a direct and formidable challenge to Mr Trimble's demand that party candidates should support the pro-agreement policy agreed by the executive committee and the ruling Ulster Unionist Council.
It is expected that Lord Molyneaux will confirm his opposition to the agreement in a broadcast interview planned for the weekend - as campaigning moves into top gear. Like Mr Donaldson, it is understood Lord Molyneaux does not plan to appear on "united unionist" platforms alongside the Rev Ian Paisley and Mr Robert McCartney. However, that offered only limited relief to Mr Trimble after the latest manifestation of UUP disunity yesterday saw the South Belfast MP, the Rev Martin Smyth, open the new United Unionist campaign headquarters in Belfast, alongside Mr McCartney and the DUP deputy leader, Mr Peter Robinson.
And it is becoming clear that the Molyneaux/Donaldson axis is wielding important influence behind the scenes at Westminster, particularly in toughening Tory demands for direct linkage between the issues of prisoner releases and Sinn Fein participation in a new Northern executive with the question of decommissioning IRA weapons.
This in turn is increasing the pressure on Mr Trimble to seek concessions from the British Prime Minister, Mr Blair, to make more explicit the decommissioning scheme defined in the original agreement, and to build in specific terms and conditions to govern a programme of prisoner releases.
Earlier unionist hopes that Lord Molyneaux would maintain a public silence were dashed by a scornful speech in the House of Lords last Wednesday. And an explicit encouragement from him now to the unionist electorate to reject the agreement would give a powerful fillip to No campaigners who are insistent they can win a majority of unionist votes. Mr Trimble has always acknowledged the crucial importance of securing a majority of unionist votes in the referendum in terms of giving the agreement broad-based legitimacy, and its impact on the psychology of the unionist parties as they enter the assembly election contest.
Under the UUP's rules, any MP wishing to seek election to the assembly requires a dispensation from the party officers or the executive committee. However, Mr Donaldson told the BBC at the weekend he did not believe the leadership would interfere in the right of constituency parties to choose their candidates. Sources last night said Mr Donaldson was "coming under strong pressure" from his Lagan Valley Association to enter the fray. And they suggested it was "inconceivable" that the leadership would try to block his nomination, despite his refusal to back the agreement. Meanwhile the UUP last night accused the Rev Ian Paisley of "running away once again" after UTV said the DUP leader would not take part in a debate with Mr Trimble which had been scheduled for Thursday.