A man told police he was raped by a stranger in Russell Square after getting lost from his friends on a night out.
Scotland Yard has put out an appeal to find a man they believe could help their investigation.
The victim, a man in his 20s, reported that he had been on a night out with friends in the Fulham area on November 26 before becoming separated from them.
In a statement police said: "He alleged he was attacked between 1am and 6am the following morning in the vicinity of Russell Square by a man unknown to him.
"He further alleged that a number of his bank cards had been stolen.
"The victim contacted police before going to hospital, and staff there too alerted police."
Detectives have released an image of a man they want to speak to.
Anyone with information about the man pictured is encouraged to call 101 or post MetCC ref CAD 4150/26NOV23.
To remain 100% anonymous contact the independent charity Crimestoppers on 0800 555 111.