A 24-year-old man from Wembley has been arrested and bailed after a woman was subjected to a sex attack on a Tube train.

Brave passengers detained a man after they became aware of the assault on a Piccadilly line service but the victim left without leaving her details.

Detectives are appealing for the woman to come forward after she was groped by a man who got on the train between Northfields and Hammersmith station on Saturday, 31 January, between 11.30pm to midnight.

Her assaulter put his arms around her as if they were couple and forced her to sit down next to him where he sexually assaulted her.

When other passengers noticed what he was doing he placed his hands over her mouth to stop her from talking.

However she managed to call for help and the man was held by passengers who called the police.

PC Paul Franklin said: “I am really keen to trace the woman who was allegedly assaulted. If you were travelling on this train and witnessed this incident, please get in touch with BTP. Any information could prove vital to this investigation.”

Anyone with information is asked to contact British Transport Police on 0800 40 50 40, or text 61016, quoting reference TSUB/B10 of 12/02/15 or Crimestoppers anonymously on 0800 555 111.

The arrested man must return to a police station on February 25.