The famously close relationship between Mr Bill Clinton and the British Prime Minister came under strain last night, as it emerged that the US President had refused two successive pleas for help from Mr Tony Blair over the Northern Ireland peace process.
In what one high-placed source described "as an extraordinary and unprecedented event", Mr Clinton rebuffed Mr Blair twice in a day during two phone calls to the White House.
On May 10th, just days after the IRA had promised to open its arms dumps to outside inspection, the Prime Minister contacted Mr Clinton, asking him to lean on both the Irish Government and Sinn Fein, to urge them to compromise on the proposed name change of the RUC.
Mr Blair was seeking White House help in extracting a republican concession on the issue.
"Clinton told Blair straight: not every issue is negotiable," an official closely involved in the peace talks said last night. "He's about the only person who could have stood up to Blair on the issue, and he did."
The President explained to Mr Blair that, while the US had sympathised with unionist demands for IRA disarmament, it had no patience with subsequent unionist demands on the RUC and the flying of the union flag on public buildings.
Mr Clinton, who has remained closely involved with the peace process, stressed his belief that the Patten report should be implemented in full "as it was printed".
The revelation comes as the Northern Ireland Secretary, Mr Peter Mandelson, is due to announce the British government's final decision on the RUC name, possibly today. He is understood to be poised to make one final move to win over unionist doubters.
According to US officials, Mr Clinton regards the notion of an overwhelmingly Protestant force in Northern Ireland as the equivalent of the mainly white police forces that ruled black areas of the US before the civil rights era.
He believes that to bow to unionist demands on the RUC "would be like leaving Alabama and Georgia under all-white cops".
The President's rebuff of Mr Blair came after an all-out effort by the British embassy in Washington to persuade US opinion-formers to understand the unionist case on the RUC. Most US political and editorial opinion had earlier sided with unionism on disarmament.
But Mr Clinton stepped in with a lobbying effort of his own, making it clear that the onus was on unionism to compromise.
The episode is the first known clash between the two leaders, who have made much of their closeness. Mr Blair backed Mr Clinton throughout the Monica Lewinsky scandal in 1998, and the President has seen the Prime Minister as his main ally in the search for a third way in centre-left politics.
But his involvement in Northern Ireland has been so intense for so long that Mr Clinton now has strongly-held views of his own. Many observers say he has often read the process more shrewdly from Washington than have his counterparts in Dublin and London.
The British government hopes the focus on the RUC may be enough to bring Mr John Taylor, the UUP deputy leader, back on-side, which would be a vital development in the push to restore devolution next week. Mr Taylor, who is in the Far East on parliamentary business, has still not made his position clear.
Concessions would anger nationalists and republicans who say there are already as many as 44 differences between Chris Patten's recommendations for RUC reform and the Policing Bill published last week.
Sinn Fein sources warned this week that the IRA initiative could be removed altogether if there were further concessions.