Though he has issued many characteristically colourful denials, few who know Boris Johnson doubt that he wants to be party leader. Ruthlessly ambitious - and sharp - beneath the Wodehousian façade, the London mayor is increasingly unable to hide his attempts to undermine Cameron. From defending City bonuses to insisting on EU referendums, the most powerful Tory in the country is a master at well-placed pronouncements that stray from the party line and are designed to appeal to the grass roots. That Cameron is suspicious of his fellow Old Etonian and Bullingdon Club member is not in doubt.
In July last year, the New Statesman revealed that relations between the two men were at an all-time low. Following denials from both sides, the BBC reported that a Cameron aide had sent Johnson a threatening text at the autumn party conference. Clashes to come could be over Crossrail, an airport for the Thames Estuary and enhanced mayoral powers. But many believe that Johnson - delusionally or not - sees his future not in City Hall, but in Downing Street.