A female passenger was sexually assaulted while she was on a train, the British Transport Police says.

Police say the incident happened on board a Thameslink service in the early hours of Friday October 22.

It has been reported between 1.45am and 2.30am, a man started a conversation with two women on-board the service running between London St Pancras and Harpenden.

A short while later, he is said to have leant forward and sexually assaulted one of them.

The British Transport Police have released an image of a man they believe has information which could assist with the investigation.

Anyone who recognises the man pictured is asked to get in touch.

Anyone travelling on the service, which would have run through Barnet and Hertfordshire, and who has any information on what happened is also asked to contact police.

The British Transport Police can be contacted by texting 61016 or by calling 0800 405060 quoting reference 2100079526.

Alternatively, call Crimestoppers anonymously on 0800 555 111.