Mayor of London Sadiq Khan said he was "extremely concerned" after footage emerged appearing to show a tram driver asleep at the controls on the same line as the crash that killed seven people.
An investigation has been launched after the footage came to light. It was filmed between the Coombe Lane and Gravel Hill stations in south London shortly after 6pm on April 21, according to The Sun.
A concerned passenger filmed the 32-second clip as the driver struggled to stay upright around three miles from the scene of the recent derailment in Croydon, the newspaper reported.
The person who recorded the film told the paper: "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, in which more than 50 people were also injured.
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 is being "urgently" investigated.