The parents of a schoolboy who fell 15ft through the top-deck window of a bus were holding a vigil at his hospital bedside last night.

The family was too upset to talk about the accident in which 14-year-old Luke Mepham suffered serious head injuries.

Inspector Simon Labbett of Sussex Police said: "There are a couple of things we have got to look into, such as what were the passengers on the vehicle doing?

"If you were seated you would perhaps not be able fall out of the window. I want to establish what the other passengers were doing, what Luke was up to and I want to look at the vehicle and establish there was nothing wrong with it."

The accident happened as Luke, from Southwater, near Horsham, was travelling home from St Wilfrid's Catholic School in Crawley on Wednesday afternoon.

The number 98 Arriva bus from Crawley was negotiating a tight turn on a mini-roundabout at the junction of Depot Road and Comptons Lane when he crashed through the window.

Luke was rushed to the Princess Royal Hospital in Haywards Heath and later transferred to intensive care at King's College Hospital in London, where his condition is described as stable.

It is not yet clear whether he fell or was pushed. Police have been taking statements from everyone who was on the bus.

Mr Labbett said: "There were lots of people on the bus. A lot of them were schoolchildren and some of them are really traumatised.

"The bus was full earlier on, but at the time of the accident we believe it had dwindled to about a quarter full."

Mr Labbett said officers were trying to establish who had been on the bus and had spoken to several schools who were compiling lists.

He said the driver of the bus was shaken by the incident.

Bernard Smith, headteacher of St Wilfrid's, said the school was deeply shocked.

He said: "Luke is a popular member of the school and the whole community is concerned and praying for him and his family."

Kevin Hawkins, commercial director for Arriva Southern Counties, which operates 17 buses from Horsham, said they were concerned about the incident and a full investigation had been launched.

Officials from the Health and Safety executive and West Sussex County Council have been informed.