Sadiq Khan has said he is "absolutely furious" after watching a video that appeared to show a London tram driver asleep at the controls just a few months after the crash that killed seven people in Croydon.
The Mayor of London said he had "demanded urgent answers" from First Group, the tram network's operator.
Footage obtained by the London Evening Standard appears to show the driver of a tram asleep as the vehicle was stopped near East Croydon.
A commuter claimed the man was asleep for "about two minutes" and that passengers were forced to bang on the window to wake him, the site said.
The driver has been suspended, First Group said.
Mr Khan said: "I am absolutely furious about this incident and I’m sure all Londoners will be as shocked as I am today.
"I have spoken to the chief executive of FirstGroup, who operate the trams, to make my feelings crystal clear.
"I’ve demanded urgent answers from FirstGroup, as well as a reassurance that every possible measure will be taken to keep passengers safe.
"I have also ordered TfL to immediately report this urgent matter to the Office of Rail and Road, and to the Rail Accident Investigation Branch.
"This comes just six months after the tragic incident in Croydon which left seven people dead and more than 50 injured. Bereaved families and survivors will be appalled by this video.
"I’d like to thank the member of the public who brought this to our attention."
First Group said: "We are very clear that it is unacceptable for tram drivers to lose concentration, for any reason, during their shift.
"We have commenced a full investigation into this incident and will take all appropriate action.
"The driver in question has been suspended from duty pending the outcome of this investigation.
"We do not tolerate any violation of our safety policies, processes and procedures. Any employee found to be in contravention of these will face disciplinary action.
"Over the past few months we have put in place additional measures to increase awareness of fatigue amongst our drivers, including individual briefings by our management, and we will continue to reinforce this.
"We are dedicated to safety and it is central to our culture and values as a transport operator. Our commitment to the safety of our passengers is unwavering, and each year our trains and buses carry 2.2 billion passengers more than 10 billion miles."