Veteran cabinet minister Ken Clarke has told Boris Johnson to "calm down" and "get back to the day job".
There has been growing speculation that Boris is positioning himself as a future leader of the Conservatives, following criticism of the government's stance on issues such as airport expansion.
But Ken Clarke told a fringe event that Boris needed to "settle down and demonstrate he can seriously deliver on some complicated subjects".
"My advice to Boris is get back to the day job and demonstrate you can be a serious leader of a local large city government and a serious deliverer of policy," he added.
Boris Johnson has used his platform speech to the Conservative Party Conference to insist he is completely loyal to David Cameron.
Boris Johnson has caused mayhem at Birmingham's main station, as he arrived in the city for the Conservative Party Conference.
Mayor Boris Johnson today warned that middle income families were feeling "utterly and understandably ignored"