Analysis: Rebels are stuck but PM is far from safe

But the former home secretary's comments are much more fascinating and subtle than a mere attack on the battered Labour leader.

His most important disclosure was that the anti-Brown bandwagon has run into the sand. "Many in the Cabinet share the view we are in great difficulty. There isn't a view ... that they should go and speak to Gordon in the way I have been describing," he said.

Mr Clarke is absolutely right in this summing-up of the current Cabinet position. From private conversations with ministers, it is clear that few seriously think Mr Brown can fully recover (though some hope that David Cameron might implode). Several concede Mr Brown has been a disappointment and a few hanker after someone younger - such as David Miliband - to take over.

But they all know, because he has made it abundantly clear, the Prime Minister would not go quietly. It would take a threat of mass resignations from the Cabinet for any delegation to be heard and Labour's rulebook makes it almost impossible to run a stalking horse challenge.

Although junior ministers were being wheeled out to attack Mr Clarke, the mood inside No10 was surprisingly chipper - seeing Mr Clarke's remarks as an admission of failure and, therefore, insignificant. For all the talk of Labour MPs signing an open letter, or of ministers resigning, the alleged ringleader seemed to say they were empty threats.

"Charles Clarke has spent the whole summer trying to get a group of MPs to write a letter or resign and he has failed," said one Brown ally. "He marched to the top of the hill, looked around and realised he was on his own."

I would not be so sure that the threat was hollow. Private conversations with Labour MPs since February have been dominated by worries about Mr Brown's performance and, increasingly, thoughts of changing leader. It calmed down in August but that's hardly surprising in a holiday and after the Glenrothes by-election was called.

It is all too easy to imagine it breaking out again ... If the latest by-election is lost in Mr Brown's own political back yard ... if Labour is still 16 points behind in the polls ... if David Cameron once again wins the party conference season ... if house prices keep falling.

Mr Clarke's admission is really a warning to ministers and ambitious MPs that they cannot sit back and expect the "Blairites" to wield the dagger alone and open a vacancy for a new Labour leader.

His public attack is a downpayment by one rebel to show his own good faith, made on the understanding that others will contribute "within months" or else enter the next election with the leadership unchanged.

Gordon Brown now has a breathing space to prove he is a winner. But the danger to his premiership has not gone away.