A pervert followed a woman through a London train while touching himself, police said.
He then allegedly exposed himself, before touching himself inappropriately.
He followed her through the train until she challenged him and he stopped, according to police.
The woman then left the train while the alleged offender is thought to have got off at Victoria station following the incident on February 20.
Officers would like to speak to the man shown in the CCTV image as he may have information which could help with the investigation.
If you know who he is, or have any information about the incident, please contact BTP by calling 0800 40 50 40 or text 61016, quoting reference number 405 13/04/17.