Boris Johnson stance puts him at odds with Theresa May and George Osborne

On the face of it, it was a typically chaotic Boris event.

Shortly before 5pm on Sunday, the London mayor ambled out of his Islington home and into the throng of waiting cameras to deliver his big announcement.

First there was there was the somewhat rambling preamble praising the wonders of European civilisation - "the greatest this planet has ever produced" - and the joys of continental holidays.

And it ended with a self-deprecatory admission - that he was joining the ranks of those seen as "crazy cranks" - of the kind that has endeared him to millions who normally take little interest in politics.

But for all the apparent disorganisation, many at Westminster were quick to detect the operation of a coolly calculating political brain beneath the most famous blond mop in Britain.

Mr Johnson might have brushed off suggestions that he was positioning himself for a tilt at the Tory leadership, but there are plenty who were not convinced.

After all, the received wisdom was that for all his flirting with Eurosceptism, he would eventually fall in behind David Cameron's re-negotiation and reap the reward with a top job - maybe foreign secretary - when his term as mayor ends.

However MPs are now wondering whether the ferociously ambitious Mr Johnson believes this could be his best chance to claim an even bigger prize.

For all the protestations among Conservatives that Mr Cameron will carry on in No 10 even if he loses the referendum, there are many who believe that he would simply be too badly damaged to do so.

In those circumstances, who would be better placed to succeed him than the most high-profile champion on the "out" cause?

And even if Mr Cameron succeeds in securing an "in" vote, there is plenty of scope for the party to tear itself apart in the process - potentially opening up the whole question of the leadership.

Again, by taking up the Brexit torch, Mr Johnson stands to garner huge support among the grassroots activists, many of whom are fiercely Eurosceptic.

Either way, when the time comes for Mr Cameron to relinquish the keys to No 10 - and he has said he will not serve another term as Prime Minister - the mayor will have carved out a distinctive position compared to rivals such as George Osborne and Theresa May.

Mr Johnson reportedly notified Mr Cameron of his intentions in a text sent just a few minutes before he stepped out into the street. It remains to be seen whether in doing so he has called time on his premiership.