British Transport Police officers have issued CCTV images of a man they want to speak to after a member of staff was assaulted at Waterloo Station.
The attack took place at around 10.15pm on Saturday 5 April on the station's main concourse after a passenger approached the member of staff asking for information.
When the man was directed towards the ticket office he became aggressive, swearing at the victim before headbutting him twice and leaving the station.
Detective Constable Ian Garden of BTP said: "This was a vicious and unprovoked assault on a member of rail staff which left him with a fractured cheekbone.
"While he has since returned to work, this has been an extremely traumatic experience for the victim and we are keen to trace the perpetrator of this crime."
After studying CCTV images from the station cameras, officers have issued several images of a man they want to speak to in connection with the attack.
The victim believes he may have had an American accent.
DC Garden said: "We will not tolerate any form of aggression against workers on the rail network.
"Staff should expect to be able to carry out their duties without the threat of violence or abuse and we will do everything we can to bring the perpetrator of this attack to justice.
"The CCTV images of the man we would like to talk to are very clear and I would urge anyone who recognises him to come forward."
Anyone with information is asked to contact British Transport Police on Freefone 0800 40 50 40 or text 61016 quoting reference SSUB/B13 of 15/5/2014. Alternatively, you can call the independent charity Crimestoppers on 0800 555 111.