Investigating officer, PC James Ainley, said: “The victim reported seeing a man carry out an indecent act while on board the train.

“She informed the train manager who called police but the man left the train at Wootton Wawen station at about 3.30pm.”

PC Ainley added: “This was an intimidating and upsetting experience for the victim. BTP does not tolerate unwanted sexual behaviour of any kind on the rail network and we take all reports extremely seriously.

“I would urge anyone who recognises the man pictured to come forward and contact BTP straight away.”

Anyone with information is asked to contact British Transport Police on 0800 40 50 40, or text 61016, quoting incident reference MSUB/B7 of 10/04/14.

Information can also be passed to the independent charity Crimestoppers, anonymously, on 0800 555 111.