An investigation has been launched after a man reportedly woke up to being sexually assaulted on a train. 

The alleged incident happened at about 7.30pm on Monday, March 4, on a Greater Anglia service to Colchester from London Liverpool Street. 

It was reported to British Transport Police that a man woke up to find a man next to him sexually assaulting him. 

Detectives have now released a CCTV photo of a man they think may have information that could help their investigation.

Anyone with any information is being asked to contact the British Transport Police by texting 61016 or by calling 0800 40 50 40 quoting reference number 325 of March 5.