Is taking on top job an Olympic challenge too far for Boris?

The Mayor of London’s case becomes more significant when anxious Tories listen to the chants of “Boris” which preceded his address to Britain’s Olympians and Paralympians following Monday’s victory parade. Enjoyed in the past by the likes of Sir Winston Churchill on VE Day and Margaret Thatcher during the Falklands war, such acclaim is unprecedented in peace time.

As I watched the scenes, a leading businessman remarked to me: “There’s the next Prime Minister.”

This, after all, is a politician who managed to raise a cheer for security staff working for embattled contractor G4S, further enhancing his man-of-the-people credentials, before telling the gathered masses: “You routed the doubters and you scattered the gloomsters.”

However Johnson already has a full-time job. He was only re-elected as Mayor of London in May by a desperately narrow margin – and this term of office does not run out until 2016. He told voters that he would serve a full term.

If he was to seek a Commons seat, a basic requirement for any party leader, he’d have to contest the 2015 election – and, in doing so, break his promise of four months ago.

As the Mayoralty is a round-the-clock, hands-on role – and because George Osborne has proved that politicians can’t hold down two senior roles simultaneously – it’s a non-starter.

If Johnson chose to do both, his credibility – and trustworthiness – would immediately become the issue, and no zip-wire would save his popularity ratings from slumping by more than six per cent.

The challenge is explaining this to the public, and fractious Tories. As Alastair Campbell, the Labour spin doctor who observed the Tony Blair and Gordon Brown power struggle from close quarters, said of Johnson’s perceived threat to Cameron: “The more he denies any prospect of challenging the Prime Minister, then as with Michael Heseltine and Mrs Thatcher, the more people will think it may happen.”