Boris Johnson's intervention  over this week's cabinet reshuffle was his most striking yet. Not only did he condemn David Cameron's decision to remove Justine Greening as Transport Secretary (an extraordinary show of dissent), he added that it would be "simply mad" to build a third runway at Heathrow and vowed to "fight this all the way", even refusing to rule out fighting a by-election  on the issue.
The Prime Minister, to put it mildly, might have hoped for a more helpful contribution from the Mayor as he sought to refresh his government. But Boris, still basking in post-Olympic glory, was determined to seize an opportunity to burnish his credentials as an alternative Conservative leader and reach out to those Tories alienated by Cameron.
It is unsurprising, then, that the Prime Minister felt it necessary to retaliate. "We will see what happens the next time he comes around with the begging bowl," one Downing Street official told today's FT . "He might need us one day." Cameron is reportedly considering withholding government support for projects such as "Crossrail II" (a new rail line from Chelsea to Hackney) and and a tunnel under the Thames at Silvertown.
If Boris and Cameron's power struggle leads to a mutually destructive war then it is Labour that will be the likely winner.