Police are asking for the public's help after a woman woke up on a train in West Wickham to find a man sexually assaulting her.

The incident happened on the 12.13am London Charing Cross to Hayes service on Tuesday April 29.

British Transport Police's Detective Constable Andy Parkinson, leading the investigation, said: "The 25-year-old victim had been out for the evening in London with friends on Monday April 28.

"She boarded the train at Charing Cross and sat in one of the last two carriages but quickly fell asleep.

"She woke up, sometime between 12.25am and 12.50am, to find a man sexually assaulting her.

"The victim managed to push the man away and fled the train when it arrived at West Wickham station a few moments later."

Police have been pursuing several lines of enquiry including analysing CCTV from the train and at several railway stations.

Today they have released CCTV images of a man they want to speak to.

DC Parkinson added: "We believe the man pictured boarded the train at Lewisham station and got off at West Wickham.

"I am eager to speak to him as I think he might have vital information.

"If you recognise him or think you may have witnessed the incident, please get in touch. We take all reports of sexual assault extremely seriously and are doing all we can to catch the individual responsible."

Anyone with information is asked to contact British Transport Police on 0800 40 50 40, or text 61016, quoting reference SSUB/B2 of 20/05/2014. Information can also be passed to the crimestoppers on 0800 555 111.

UPDATE: Man arrested after West Wickham train sexual assault appeal