Police want to speak to this man after a train passenger performed a sex act in front of a schoolgirl.

British Transport Police have released two pictures following the indecent act on a train travelling between Kent and south-east London.

It comes after a man and the 15-year-old girl separately boarded the 4.01pm train to Cannon Street at Dartford on May 23.

DC Andy Parkinson said: “After boarding the train, the man sat opposite the girl, and began touching himself inappropriately while staring at his victim.

“He continued this behaviour until Woolwich Arsenal station, where he got off the train before catching another back towards Dartford.

“Since the incident we believe the same man has been sighted at Erith station.

“This was a frightening experience for the girl and we need to find the man responsible.”

Police have now released a CCTV image from the train - and another one taken by the quick-thinking girl.

DC Parkinson added: “We treat all forms of unwanted sexual behaviour on the rail network seriously and are following several lines of enquiry in this case."

Anyone with information should contact British Transport Police on 0800 40 50 40, or text 61016, quoting reference SSUB/B4 of 03/07/2014.