A CCTV image of a man wanted for questioning over a sex offence has been released.

The offence occurred on a train traveling from Gravesend to Charing Cross station last year.

The victim, a 17-year-old girl was approached by a black man at Gravesend station at around 2.30pm at Gravesend station on December 17.

The man sat next to her as she waited for a London-bound train.

British Transport Police (BTP) Detective Sergeant Paddy Kerr said: "When the train arrived, the male followed the girl onto the carriage and sat next to her, putting his arm around her.

"She asked him to remove it but he would not comply.

"The male then sexually assaulted the complainant and would not allow her to leave.

"She eventually managed to break free from the male and got off the train at Swanscombe."

DS Kerr said police had followed a number of inquiries and hoped the release of the CCTV image may result in a member of the public identifying a male suspect.

He added: "The male had a medium to large build, muscular or athletic build.

"He was 6ft tall, mid 20's, with black Afro style hair, no facial hair, a scar above left eyebrow and a large drooping bottom lip.

"He is believed to have a London accent, which at times was African.

"He was wearing a black hooded top with red writing across the chest and grey fur lining on the inner hood.

"Police believe he may have worn two hooded tops to make him look bigger. He was also wearing dark coloured jeans and white trainers.

"We would appeal to anyone who could assist us in identifying this male to contact us on 0800 40 50 40 or by calling 0207 391 5275 and quoting background log B4 of 6/12/07."