Boris Johnson didn’t need to be disloyal to David Cameron in his speech to the Conservative conference today and he wasn’t. He was merely very discourteous. Picking up reports that the Prime Minister had referred to him as a “blonde mop” he repaid the compliment in vigorous back-handed style. Cameron, he said, is a “broom” sweeping up Labour’s mess. George Osborne, he added, is a dustpan.

Johnson knows Cameron well enough to understand that nothing gets under his skin quite like lèse majesté. The informality, picking the Prime Minister out of the crowd, calling him “Dave” and wishing him a happy birthday with a wilful lack of deference will have been exquisitely irritating. The  over-chummy manner of delivery contained a deadly whiff of ridicule. The Mayor of London didn’t attack the Prime Minister on policy but nonetheless found a way to diminish his stature. 

That served the underlying purpose of Johnson’s speech which was to present himself not necessarily as a current rival to Cameron but as his equal nonetheless. The bulk of Boris’s speech was a celebration of his record in running London with an emphasis on economic vibrancy. He talked about creating a “platform for growth” – developing young people’s skills, finding them jobs, developing infrastructure, boosting exports and attracting investment. It was optimistic in tone and ambitious in scope, yet cleverly contained in the Mayor’s own geographical remit.

There can be little doubt what the objective was here. Boris is setting up his record of governing London as an audition for one day running the country. He was rehearsing a celebration of what can be achieved in the capital – both in terms of beating Labour and kick-starting the economy – as a blueprint for how Conservatives should feel more confident about what they can achieve as a national party. (Whether or not his record will ever justify such exuberance is an entirely different matter.)

He said nothing that sounded like an explicit threat to Downing Street and the Prime Minister’s aides affect to be pointedly relaxed about Boris’s ambitions. The Number 10 line is that Johnson will obviously serve his full term as Mayor, by which time the next election will already be decided. If he wants to do something after that – enter parliament or aspire to be leader – it is a matter for future conjecture that is, in political terms, so distant as to be unworthy of further comment.

Privately, Number 10 sources argue that if Cameron wins the next election, he stays on as leader. If he doesn’t he will almost certainly step down and then Boris might or might not engineer a way to make himself a candidate for the succession. Either way, a straight Boris v Dave contest will never happen. Ergo, Johnson is not a threat to Cameron.

It is a plausible argument but one that ignores the slow drip effect on party morale and Prime Ministerial authority of having, in the wings and periodically intruding on stage, an ebullient, popular Tory figurehead who pointedly refuses to genuflect before the leader. At the moment, Downing Street’s approach is to ignore Boris and laugh along through gritted teeth. Before long, Cameron will surely feel the need to find a more active strategy for cutting the London Mayor down to size.