THREE people are feared dead today and 12 injured after a passenger train derailed, smashing into a bridge and a station platform at Potters Bar, north of London.

The four-carriage 12.45pm West Anglia Great Northern service was near Potters Bar station in Hertfordshire when the accident occurred just before 1pm.

Herts police said there were unconfirmed reports of fatalities.

The ambulance service said early reports suggested a "significant number of casualties". The Fire Brigade was preparing to rescue trapped passengers.

The train had just left King's Cross station bound for King's Lynn in Norfolk.

The derailment was within a few minutes running time of Hatfield, where a GNER express came off the tracks in October 2000 in an accident which claimed four lives.

Railtrack reported three of the train's four carriages were derailed and the final carriage hit the platform at Potters Bar.

It was not immediately known if anyone on the platform was hurt.

Hertfordshire Police said in a statement: "Reports have come into Hertfordshire Constabulary of a serious rail crash near Potters Bar railway station just before 12.55pm.

"This is being treated as a major incident and we are responding as a matter of urgency.

"There are reports of fatalities although there is no confirmed information at this early stage."

Several ambulances were sent to the scene. An air ambulance was also sent. The Bedfordshire and Hertfordshire Ambulance Service asked the neighbouring London Ambulance Service for help in coping with the emergency.

A Health and Safety Executive spokesman said: "Our Railway Inspectorate team are looking into this incident and are expected to travel to the scene soon."

Hertfordshire Fire Brigade said there were reports that some passengers were trapped on the derailed train.

Eight fire engines were originally sent to the scene of the incident after reports that there were a number of casualties.

London Fire Brigade said it had sent three appliances including a fire rescue unit, which includes cutting gear, to assist the operation.

Witness Nicholas Anastasiou told Sky News he heard a "big bang and that was it."

He said: "One carriage is very badly damaged indeed. In the car park there are bits of the train everywhere.

"People are quite badly hurt. There are loads of hurt people here.

"There are people running around trying to help people but I don't think they can do much because it's quite bad. All I can see is the last carriage."

Prakash Maehta said he had heard "a bang", adding: "There might be quite a lot of people injured."

He said emergency services had poured into the area.

"The last carriage is practically on the platform," he added.

Another eyewitness said it appeared the train had become derailed after crossing under a road bridge close to the station.

Debris from the bridge had fallen onto Darkes Lane, one of the main roads in the town.

It is believed that debris also covered the railway station car park.

A Sainsbury's store near the scene was being used as a first aid post, the company said.