Mr Ken Maginnis raised the stakes in the peace process last night, telling the British government it would have to talk directly to the IRA if Sinn Fein maintains it cannot commit the republican movement to decommissioning.
The Ulster Unionist security spokesman made his intervention as the Taoiseach, Mr Ahern, and the British Prime Minister, Mr Tony Blair, prepared to return to Northern Ireland for three days of negotiations which could make or break the Belfast Agreement.
Piling the pressure on Mr Blair, who insists Wednesday's devolution deadline is "absolute", Mr Maginnis said any undertaking on decommissioning given this week by Sinn Fein purely on its own account would be "worthless".
And in a further reminder of Mr David Trimble's limited room for manoeuvre, the Fermanagh MP made it clear he would not support any move to form the Northern Ireland executive without a "simultaneous" start to decommissioning by the IRA.
In what will be seen as a further entrenchment of the UUP position following Mr Jeffrey Donaldson's return to Mr Trimble's talks team, Mr Maginnis said: "I will not be accepting any post-dated cheques or promises."
Describing himself as "totally frustrated by, and sceptical of, Sinn Fein's repeated insistence that it does not speak for the IRA", Mr Maginnis said: "It really is time to challenge this illusion. We need to know who speaks for the Provos, and we need to know what they have to say on this issue."
His comments came just hours after Sinn Fein's chief negotiator, Mr Martin McGuinness, told the BBC's On the Record television programme that decommissioning was purely a matter for the IRA.
Mr Maginnis told The Irish Times that any commitment given by Sinn Fein to Mr Blair and Mr Ahern in respect of decommissioning within the time scale envisaged by the agreement - that is by May 2000 - "would have to be by Sinn Fein on behalf of the IRA, otherwise it is worthless, and the government is talking to the wrong people."
Mr Maginnis said: "If these people, McGuinness, Doherty, Ferris, Keenan and Adams, are not able to speak for the IRA then it's time we realised government has got to identify, and deal directly, with `P. O'Neill'." And he continued: "If they're really telling me they cannot commit the IRA, then their role within the terms of the agreement means nothing, and government must identify those who can."
The Irish Times has learned that the Ulster Unionists are insisting that the timing of any IRA decommissioning should not fall within the authority of the International Commission on Decommissioning, which is due to present a report to the parties tomorrow.
This issue was raised during the UUP's first meeting with Mr Blair and Mr Ahern on Friday, when the leaders sought the party's acceptance for the subsequently announced three principles, the third of which provides for "decommissioning to be carried out in a manner determined by the Independent Commission".
While Mr Blair reportedly indicated the "manner" would not include the question of timing, this was subsequently denied by a Northern Ireland Office source.
The UUP will return to this issue when they meet Mr Blair and Mr Ahern this morning. Mr Trimble is also expected to press the Prime Minister and the Taoiseach on Mr Blair's offer in Friday's article in the London Times of "a cast-iron, fail-safe device" to collapse the executive if decommissioning has not been completed by May 2000 and to define what political arrangements would follow.
In an article in the Sunday In- dependent yesterday, Mr Maginnis attacked Mr Blair's "arbitrary June 30th deadline" and warned that four years of endeavour would be "wiped away at the whim of a tired and overwrought Prime Minister who has for the past few months, on top of the usual national and international pressures, had to burn the midnight oil over Kosovo and a European election which went depressingly wrong for his party".