A tram which derailed in south London killing at least five people was travelling at a "significantly higher speed than is permitted," according to the Rail Accident Investigation Branch.
Fifty people have also been injured in the crash and taken to hospital, with four said to be seriously injured.
Transport for London said the tram derailed just after 6am. The RAIB is still at the scene investigating the cause of the accident.
Police have set up a cordon spanning at least 200 metres whilst investigations continue.
British Transport Police have arrested the driver.