The story was damaging to the PM because any suggestion that he has even contemplated chucking the job in inevitably weakens him. The fact that the two biggest hitters who went to Downing Street to tell Blair not to quit were ministers who would be lucky to get a cleaning job in Number 10 if Brown was in charge ensured that the story had plenty of 'legs'.
It is significant that two cabinet ministers who both have leadership aspirations, David Blunkett and Jack Straw, kept quiet.
Inevitably, some of the political disciples who hang around Blair blamed Brown for leaking details of private meetings between the PM and his cabinet colleagues. According to The Guardian, some of them even suggested that I was behind a campaign to destabilise the PM!
Brown was just as upset as Blair about Marr's story. The last thing he needed as he prepared for his big spending statement that will determine the dividing lines between the parties for the next election was yet another round of Brown/Blair split stories.
Downing Street comms director David Hill moved quickly to tell anyone who would listen that Blair did not think that Brown was involved in the story. But The Independent ran a 'New rift opens between Brown and Blair' headline the next day.
Brown believes that Blair will fight and win the next election and that any undermining of the PM by himself or his allies will not be forgiven by the Labour Party, which he hopes will one day elect him as leader.
And after Monday's performance, who can doubt Brown's leadership abilities?
The Sun may have led the news on the eve of his big speech with a story that Blair intends to go on forever, but what else can he say?
The Chancellor looked at his confident best, especially when responding to the lacklustre Oliver Letwin who kept going on about 'fat government'.
Does he mean that Brown should lose weight? The shadow chancellor did at least mention 'third-term tax rises' but Brown's spin doctors had been busy. Even before he sat down, I picked up a copy of the Evening Standard which splashed 'Brown's big tax pledge'. This said that he had ruled out tax rises even though there was no mention of tax in his speech. Let the battle commence.