Three of the train's four carriages derailed in the accident which happened just before 1300 BST.

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As they slewed across the track on the approach to the station a road bridge was damaged and later partly collapsed.

The rear carriage flipped across two platforms and became wedged under the canopy of the station roof.

The train was not due to stop at Potters Bar and it is thought it was travelling at up to 100 mph.

Chief Superintendent Andy Wright, who is leading the rescue effort, compared the devastation at the scene with the Hatfield crash two years ago, in which four people died.

'Like a bomb'

He said all the dead and most seriously injured had been in the rear carriage.

Eyewitness John Fuller described the carriage being "spread-eagled" across the tracks.

The carriage that is underneath the station's canopy is totally destroyed - its metal is twisted

Paul Davies Eyewitness

"It looks like a bomb explosion - there's very bad damage," he said.

The driver of the West Anglia Great Northern train, Andy Gibson, from London, was uninjured and said he "felt the train give way from the rear of the train".

A spokesman for his union Aslef told the BBC he felt the train - which had 151 people on board - had passed over something.

'Panic'

Andy Perveris was waiting on the platform when the crash happened.

"I heard a bang, turned round and saw a train carriage on the platform coming towards me," he said.

Andy Perveris ran as the train careered towards him

"Everyone just turned and legged it.

"I saw bodies lying on the rail, heard screaming and saw smoke and dust settling from the train.

"The first thing I did was panic. The second thing was get onto the rail to see if there was anyone who needed help."

Mr Perveris went to the aid of an unconscious woman. She died in his arms.

Carriage 'destroyed'

Eyewitness Paul Davies said he could see the station from the window of his office.

The train shuddered and seemed to catapult

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"The carriage that is underneath the station's canopy is totally destroyed.

"Its metal is twisted."

The injured were taken to Barnet General Hospital.

A spokesman said injuries ranged from open fractures to head and chest injuries.

Investigation

The Queen was said to be "shocked" to learn of the crash.

Transport secretary Stephen Byers ordered an investigation by the Health and Safety Executive.

"All of our thoughts must be with the families and friends of those who have been killed or injured today in this tragic accident," he said.

Debris from the bridge hit by the train fell onto vehicles

Several News Online users and regular travellers on that line said there was a noticeable "jolt" every time the train went through Potters Bar.

"I have been waiting for this the last couple of weeks," said Tim Richardson.

"Every time we pass that area the carriages have been hitting something on the rail and shaking violently."

Recent rail disasters

Great Heck 28 Feb 2001 Hatfield 17 Oct 2000 Ladbroke Grove 5 Oct 1999 Southall 19 Sep 1997 Watford Junction 8 Aug 1996 Clapham Junction 12 Dec 1988

At a press conference on Friday evening, Railtrack chief executive John Armitt said the maintenance record of the track, and the last set of points prior to the crash, would be examined.

George Muir, of the Association of Train Operating Companies, told BBC News: "There's been an extraordinary focus on safety in the last two years, which I hope passengers understand.

"The investment that's gone into safety is very much more than ever before - £3bn last year on renewals and maintenance."

Potters Bar station is only a few miles down the line from the site of the Hatfield train disaster in which four people were killed two years ago.

Louise Christian, the solicitor acting for the victims of the Southall and Ladbroke Grove disasters, called for an immediate public inquiry into Friday's tragedy and the Hatfield crash.

Emergency information 0845 944 1551