Officers have released CCTV footage of the man following the incident, which took place between 1.30am and 2.30am on Thursday, July 24.

Police say the 23-year-old woman fell asleep on her way back from a night out in London, and woke up to find a man touching her.

Detective Constable Steve Eyers said: “A man boarded the train at London Paddington and sat opposite the woman. During the journey, the woman fell asleep and awoke to find the man touching her.

“Any form of unwanted sexual behaviour is extremely disturbing and we are committed to tackling these offences in whatever from they take.

“Thankfully crimes of this nature are rare on the railway but, when they do happen, they are all the more frightening because of the relatively confined space.

“Investigations to date have allowed us to isolate CCTV images of a man we want to speak to as we believe he will have information which can assist the investigation. He got off the train at Reading station.”

Anyone who recognises the man should call the British Transport Police on Freefone 0800 40 50 40 or Crimestoppers, anonymously, on 0800 555 111, quoting reference number GSUB/B5.