Police have launched a murder inquiry after a man died from a punch in the face.

David Illingworth, 23, was knocked backwards by the blow and hit his head on a road.

Police said Mr Illingworth, a former Sussex University student and more recently a fork-lift truck driver, was walking with his girlfriend when a dispute broke out with a group of three people, one a woman.

Police said a punch was thrown and Mr Illingworth hit the ground.

He was taken to the Royal Sussex County Hospital, Brighton, and later transferred to Hurstwood Park neurological hospital in Haywards Heath, where he died on Thursday.

His mother and father, who live in Northamptonshire and Manchester, were at his bedside.

Relatives and his girlfriend of six months, an administration clerk, are said to be distraught.

Mr Illingworth, who lived with his girlfriend in London Terrace, off London Road, Brighton, was in Ship Street, near the junction with North Street, Brighton, at the time of the fight at 3am last Saturday.

Police returned to the scene last night in the hope of tracing witnesses. Two men arrested have been released on bail pending further inquiries.

Detective Chief Inspector Martyn Underhill, who is leading the inquiry, said: "This appears to be a one-blow incident which had tragic consequences."

Mr Underhill said it was vital witnesses came forward.

He said: "It was 3am on a Saturday morning and in that part of the city there are still plenty of people around and vehicles driving by.

"We are sure someone saw something and I would urge them to come forward.

"We are treating this as a murder inquiry. This was an extremely serious assault and we need as much help from the public as possible."

He appealed to three people who helped the victim at the scene to come forward.

One was a man of Hispanic appearance, 5ft 8in, in his 20s, who was wearing a denim jacket. It is believed he lives and works locally.

Two other men, one with a pedal cycle, were white.

Anyone with information should call Brighton and Hove CID on 0845 6070999.