A murder investigation has begun after a Sri Lankan man who was attacked outside a Tube station died in hospital yesterday.

Police said Vipaul Prasanna, 41, was pronounced brain dead about 6pm, leading to the decision to turn off his life support machine at the Royal London Hospital in Whitechapel.

Mr Prasanna, who was working in Brixton to send money back to his wife and children in Sri Lanka, was found lying unconscious outside the Wembley Park Tube station just before 11am on Sunday.

He had been involved in an argument with a man who punched him in the head, causing him to collapse to the ground and hit his head on the pavement.

Police said the argument started as Mr Prasanna left the station, when the man tried to push his son through a ticket barrier behind him.

Mr Prasanna is thought to have trod on the boy's foot and, although he was seen apologising, was hit with an uppercut to the chin.

The man and his son, who got on the Metropolitan line at Rayners Lane before getting off at Wembley Park, were last seen walking toward the Chalkhill Estate.

He is described as a black man in his twenties with a powerful build.

At the time of the altercation he was wearing a white long-sleeved top with dark trousers and a dark baseball cap.

His son, aged about ten years old, was wearing a camouflage coloured cap, with his hair sticking out the back.

Detective Inspector Ian Foster said a minor disagreement had escalated into violence against a stranger that had resulted in the death of a committed family man.

"It is acknowledged that the man who carried out this attack may not have intended for his actions to have such serious consequences, but it is in his own interests for this man to get in touch with police, and this has not yet happened."

Mr Foster said detectives had already studied CCTV and spoken with several witnesses, but wanted more assistance in finding the man who had hit Mr Prasanna.

Information can be passed to police on 0208 358 0200 or, to remain anonymous call Crimestoppers on 0800 555 111.