Peter Mandleson quipped last week that one of the things he had in common with Boris Johnson was that they both wanted to undermine David Cameron.

Johnson's latest contribution to this is in The Telegraph today where he says the 50p rate of income tax would be a disaster for Britain. Apparently this "disaster" would mean the loss of Tracey Ermin, Michael Caine and a number of other wealthy folk who would move to their other homes in warmer climes.

Now, let us not dwell too long on the sadness that the loss of Tracey Ermin will bring to households across the country, or whether Michael Caine would finally leave for good (he has been threatening this before every general election for the last 30 years and the bugger is still here). Nor linger unduly on the merits or otherwise of the 50p tax rate. I have a friend who arranges investments for a number of high earning Premiership professional footballers, and when I asked him whether the 50p tax rate would send them scuttling to Barcelona, Lyon or Milan he scoffed and said, "Get real. You don't think these people pay that sort of tax do you? That's why they have accountants and people like me."

No, the real story is that Gideon Osborne and Cameron have hinted very heavily that scrapping the 50p tax rate was not high on their agenda... and Boris Johnson knows it. The Telegraph article is a typical piece of Johnson mischief making. Mandleson wasn't joking about the tension between the Bullingdon Boys.

Of course, Blair coped with a London Mayor who, let's be kind, he didn't always see eye-to-eye with. But Blair had a massive Commons majority behind him, and all of the authority that three landslide elections bring with them. If Cameron should win with the sort of paper thin majority being predicted by many, Boris Johnson won't be so easily dismissed.