A schoolboy was recovering in hospital today after being thrown head-first through the window of a moving double deck bus.

The 14-year-old suffered serious head injuries after plummeting 15ft on to the pavement.

He was slipping in and out of consciousness at the roadside as paramedics raced to the scene.

He was taken to Princess Margaret Hospital in Haywards Heath before being transferred to the paediatric intensive care unit at King's College in London.

Bus firm Arriva has launched an investigation to discover how the accident happened.

The boy is believed to be one of a group of children playing on the top floor and jumping on seats as the 98 bus, carrying schoolchildren and members of the public, made its way from Crawley to Horsham at 4.20pm yesterday.

Initial reports suggest the bus lurched as it negotiated a tight right-hand bend at the junction of Depot Road and Comptons Lane, Horsham, sending the youngster flying.

Kevin Hawkins, commercial director for Arriva Southern Counties, which operates 17 buses from Horsham, said: "We are extremely concerned and sorry to hear what happened.

"It is not something that is common. It is extremely uncommon for this to happen."

Mr Hawkins said the company had launched a full investigation into the incident.

Police were today interviewing the boy's friendsas they tried to find out how he had fallen.

Inspector Simon Labbett said: "We are keeping an open mind. This will be a far reaching investigation looking at both the vehicle and the potential actions of the passengers."

The schoolboy is said to be in a stable condition and his injuries are not life-threatening.

The Health and Safety Executive and education authority have been informed.

A police spokesman said the bus had been relatively empty at the time and most passengers were schoolchildren.