Police have released CCTV images of a man who allegedly exposed himself and masturbated in front of a Spanish woman on board a Tube train.
The man is alleged to have boarded a Jubilee Line train at Green Park at around 11.40pm on Friday, September 12, and sat opposite the woman.
He then followed the woman after she got off at Dollis Hill, according to police.
The victim alerted staff at the station, and the man then got back onto the same train and left the station, it is alleged.
Detective Constable Allan McTaggart, of the British Transport Police's sexual offences unit, said: "I am urgently trying to trace the man pictured, as I think he might be able to greatly assist the investigation. If you recognise him, or have any other information, please get in touch with BTP as soon as possible."
Anyone with information is asked to contact British Transport Police on 0800 40 50 40, or text 61016, quoting reference TSUB/B3 of 16/10/2014. Alternatively, people can call Crimestoppers anonymously on 0800 555 111.
