“The thing about Boris Johnson is despite all the clumsiness and bumbliness he’s actually a really, really ambitious politician,” Mr Clegg said.

“He treats his political ambition like he treats his hair. He wants everybody to think he doesn’t really care, but he actually really, really does care.

“His tousled hair, his bumbliness, all that’s great. But behind all of that is someone who is absolutely fixated with his own political ambitions.”

Mr Johnson is neck-and-neck with Mr Osborne among Tory activists, a poll today shows.

Boris Johnson has a net approval rating of 83.3, marginally ahead George Osborne of George Osborne on 83, making them the two most popular senior Tories, according to a survey of 900 party members by Conservative Home and YouGov. By contrast, David Cameron sits in the middle of the table with a net approval rating of 48.7.