A Croydon tram driver has been suspended as part of an inquiry into video footage that appears to show a driver falling asleep at the controls.
The clip, captured on a passenger's mobile phone on Wednesday, was sent to the Evening Standard newspaper.
The video was recorded three stops from the site of the 9 November derailment in Croydon that left seven people dead and 51 injured.
Mayor of London Sadiq Khan said he was "absolutely furious" about the footage.
In the 20-second clip, a driver - who is not the one involved in last year's fatal crash - appears to be sleeping while the tram is waiting at lights.
When the lights changed, he failed to move off.Image copyright EPA Image caption
A commuter told the Evening Standard the driver was asleep for several minutes and people resorted to banging on the window to wake him up.
He is seen bent over and breathing heavily with his hands still on the controls.
Mr Khan said he was sure Londoners would be "as shocked as he was" about the incident, and called for an immediate investigation.
"I have spoken to the chief executive of First Group, who operate the trams, to make my feelings crystal clear.
"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."Image caption
A Transport for London spokesman said it had asked operators First Group to take all necessary action to ensure they are dealing with driver tiredness.
First Group said in a statement: "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."
Following the 2016 derailment, four drivers told the BBC that they had fallen asleep while operating trams in Croydon.
An interim crash report found the crashed tram had been travelling at 43.5mph in a 12mph zone and found no evidence of any track defects or obstructions.
The driver was arrested on suspicion of manslaughter and is currently on bail.