A man has died while working on a moving walkway at Waterloo station.
Paramedics were unable to save the 44-year-old engineer, who has yet to be identified, and he was pronounced dead shortly after 02:20 BST.
British Transport Police officers are investigating the death, which is being treated as unexplained.
Vernon Everitt, London Underground's managing director, expressed "deepest condolences" to the man's family from the rail network.
"We are also very conscious of the impact this sort of incident has on first responders and station staff, and a full support network has been stood up," he added.
Police confirmed the man, from Cambridgeshire, was a contractor and was injured while working at the Underground station.
Passengers were advised they would be unable to change lines because of a fault with one of the Tube station's two travelators.
A one-way system was implemented during rush hour on Wednesday morning.
Det Insp Darren Gough said: "This is a truly tragic incident and our deepest condolences are with the man's family.
"They are currently being supported by specially-trained family liaison officers as they come to terms with this devastating news."
Mayor of London Sadiq Khan called for an urgent inquiry into what happened.
He said on Twitter: "Very sad to hear of the tragic death of a contractor at Waterloo Station this morning.
"I know I speak for everyone at City Hall and TfL in sending our deepest condolences to their family & friends."