Right-wing political activist Tommy Robinson has fled Britain after an investigation was launched into an alleged assault on a man at St Pancras Station, police have confirmed.

Detectives have launched a manhunt for the 42-year-old, from Luton, Bedfordshire, after he boarded a flight out of the country in the early hours of Tuesday morning.

His flight out of Britain comes just hours after footage circulated online showing him pacing back and forth beside the unresponsive man lying face-down on the station concourse.

Paramedics rushed to St Pancras Station, in central London, at around 8.40pm last night and the alleged victim was taken to a major trauma centre with 'serious injuries'.

In a video shared widely online, Robinson, whose real name is Stephen Yaxley-Lennon, appeared to be walking in the background as security were called to help the unconscious man.