An investigation has been launched after footage emerged appearing to show a tram driver asleep at the controls on the same line as the crash that killed seven people.
A concerned passenger filmed the 32-second clip as the driver struggled to stay upright around three miles from the derailment in Croydon, south London, The Sun reported.
Mayor of London Sadiq Khan said he was "extremely concerned" by the video and called for a full investigation.
The footage was filmed between the Coombe Lane and Gravel Hill stations shortly after 6pm on April 21, according to The Sun.
The person who recorded the film told the newspaper: "It was the most surreal thing because the tram is zooming along and the driver seems to be falling asleep.
"If you are in charge of 50-plus lives and you're that exhausted, it's a dangerous thing."
The footage surfaced as the tram line, operated by First Group and owned by Transport for London (TfL), reopened nine days after the fatal derailment, which injured more than 50.
Mr Khan, said: "I am extremely concerned by this video. People will understandably be very worried.
"It is important that First Group, who operate the trams, and TfL urgently and fully investigate this. I will make sure any ?necessary action is taken, and that all steps are taken to make sure these trams are as safe as possible."
TfL said the footage was being "urgently" investigated.
A First Group spokesman added: "We have not seen this video before and it will now be subject to a full investigation. If the situation is as it appears then this is completely unacceptable and appropriate action will be taken."
Investigators said the derailed tram was travelling at three-and-a-half times the speed limit when it crashed near Sandilands station on Wednesday morning last week.
The tram's driver, Alfred Dorris, 42, from Beckenham, south-east London, was arrested at the scene and questioned on suspicion of manslaughter before being bailed until May.
The seven killed were Dane Chinnery, 19, Philip Logan, 52, Philip Seary, 57, Dorota Rynkiewicz, 35, and Robert Huxley, 63, all from New Addington, and Mark Smith, 35, and Donald Collett, 62, both from Croydon.
The Rail Accident Investigation Branch (RAIB) said the tram, which was carrying about 60 people, was doing 43.5mph in a 12mph zone.
In its interim report, it said there was no evidence of any track defects or obstructions on the track, and the investigation also found no malfunction of the braking system.
Engineers have repaired the track and other equipment and have run trams over the repaired section.