Boris said: "I just want to ask you when are you going to get all those Government ministers out of their posh limos and on to public transport like everybody else?"
The jibe comes after Transport Minister Simon Burns was criticised last year for overseeing inflation-busting rises in rail fares, while spending £80,000 a year on a chauffeur-driven car to travel 35 miles to Westminster from his home in Chelmsford.
David Cameron announced a crackdown on ministerial cars, which cost more than £10million a year under Labour, when he came into office.
He said: "If there is something that really annoys people it's seeing politicians swanning around in chauffeur-driven cars like they're the Royal Family."
Watch the video below to see Nick Clegg's response to Boris Johnson's curveball questioning