Nick Clegg has mocked Boris Johnson as a "conventional" politician obsessed with his own hair and quest for power.
The Deputy Prime Minister said the London mayor liked to pretend he did not care about his appearance or career - but was actually "fixated" with them.
The comments, on Mr Clegg's regular LBC radio phone-in, came after Mr Johnson ended years of speculation by confirming he will seek to return to Parliament at the general election.
Many believe he is positioning to take over from David Cameron as Tory leader, although the mayor himself has dismissed the idea as "highly unlikely".
Mr Clegg said: "The thing about Boris Johnson is despite all the kind of clumsiness and bumbliness, he's actually a really, really ambitious politician...
"He treats his political ambition a bit like he treats his hair. He wants everyone to think that he doesn't really care, but he actually really, really does care."
The Lib Dem leader said Mr Johnson had until now been able to "have his cake and eat it".
"He will have to come clean about the fact that he is in many ways a much more conventional politician than he likes to appear," Mr Clegg said.
"His tousled hair, his bumbliness, his humour, all of that is great... all I am saying is behind all of that is someone who is absolutely fixated with his own political ambitions. In that sense, he is actually a very conventional politician.
"In a sense, being a mayor is great, a wonderful position to have, but you can kind of have your cake and eat it. You can sort of lob grenades into the political debate without having to take responsibility for stuff.
"At some point he is actually going to have to say 'I'm going to do responsible stuff, I've got to take difficult decisions'."