Labour has turned on him, claiming the Tories are in disarray after London Deputy Mayor Ray Lewis was forced to stand down following claims of sexual and financial misconduct.

Labour is in no position to accuse anyone of presiding over a shambles – not after 10 years of incompetence and borderline corruption.

But, nevertheless, Mr Cameron should heed the warning signs.

In recent months luck has gone his way, the polls have been kind and Gordon Brown has made Cameron appear a credible leader-in-waiting.

But what goes up can also come down. If London Mayor Boris Johnson is the advance guard of a Tory march on Downing Street then he must prove that they are up to the job.

The highest office requires the highest standards of judgment. Mr Cameron and his lieutenants must show the electorate they are worthy of our trust.