Deputy Assistant Police Commissioner Andy Trotter said another 100 people, who may have boarded one of the trains, were also unaccounted for.

BBC News' Ben Brown reports on the day's activities at the crash scene

The crash happened at 0811 BST on Tuesday, when the 8.06 three-carriage service run by Thames Trains from Paddington to Bedwyn in Wiltshire collided with the London-bound 06.03 Great Western 125 express train from Cheltenham at Ladbroke Grove.

A BBC correspondent says the initial Health and Safety Executive investigation suggests the Thames train passed one red and two yellow warning signals, ploughing into the Great Western, which had been given a green light.

Railtrack has confirmed the signals were working properly at the time.

Mr Trotter said: "Yesterday's train crash was a terrible tragedy, the scale of which is only just becoming apparent."

He said 70 people who had definitely boarded one of the two trains were missing. Another 100 were unaccounted for, although their relatives and friends were unsure whether they had boarded one of the trains.

Mr Trotter said Scotland Yard's casualty bureau had received 5,000 calls from people concerned about relatives or friends.

BBC News' Robert Hall: Casualty toll expected to rise significantly

Eighteen bodies have been removed from the crash site but many of the victims may be unidentifiable because of the intense heat generated by fire in the first class carriage.

Chief Fire Officer Malcolm Kelly, speaking at a news conference at New Scotland Yard on Wednesday, said temperatures within the carriage may have reached 1,000°C.

Mr Trotter said: "It will be extremely difficult to identify anything else in that burnt-out carriage."

He said a team led by a Home Office pathologist would begin a fingertip search of the first-class carriage on Thursday.

The BBC's Clarence Mitchell: "The task of removing the dead continued throughout the morning"

Mr Kelly said a 100-ton crane would be on the scene shortly but he said the carriage was so fragile, because of the crash and the ensuing fire, it may break up if they tried to lift it.

A criminal investigation into the cause of the accident is being supervised by Superintendent Nick Bracken, of the British Transport Police.

It has emerged that the driver of the Thames Train had been in the job for only two months, following 11 months of training.

Meanwhile, train drivers are to be balloted on strike action unless rail companies respond within seven days to demands for a package of improved safety measures.

The train drivers' union, Aslef, is demanding the nationwide installation of a fail-safe automatic train protection system as well as in-cab radios.

Tuesday's disaster at Paddington is set to eclipse the 1988 Clapham crash - in which 35 people died - with many bodies believed to be trapped in a carriage which was gutted by fire.

A firefighter takes a break during the search for bodies

Forty-six people remain in six London hospitals - 10 in intensive care units and another six in specialist burns units.

An urgent public inquiry has been ordered by Deputy Prime Minister John Prescott, who has also demanded a full review of train safety.

The Paddington comes only two years after seven people died and 150 were injured at Southall, barely eight miles away.

The public inquiry only began last month, and critics say valuable safety lessons may not have been learned.

Tuesday's crash was described as "almost a replica" of Southall, caused by a train going through red signals. It took place on the same stretch of line.

The Prime Minister, Tony Blair, and his wife Cherie visited St Mary's Hospital in Paddington, where many of the injured are being treated, on Wednesday.

Mr Blair thanked the emergency services for the "extraordinary" way they had responded to the disaster.

The BBC's Mike Embley interrogates Railtrack's Director of Operations Chris Leah about rail safety

Later the Prince of Wales visited the hospital and expressed his condolences to the families of the victims.

US President Bill Clinton sent his condolences, saying: "The incident is particularly tragic because it happened in such an everyday setting - as commuters headed toward another day at work."

Some of the rescue workers are to be sent home because they have been traumatised.

Special centres have been set up where people concerned about relatives can provide and seek information about their loved ones.

Counsellors will be on hand at the four centres: The Thistle Hotel in Swindon, Wiltshire, The Thistle Hotel in Cheltenham, Gloucestershire, The Royal Lancaster Hotel in Bayswater, central London, and the Town Hall in Reading, Berkshire.

Click here to see the site of the crash

Police have not named any of the victims, but it is understood the two train drivers were among those killed.

A black box recorder has been recovered from the Thames train, although the Great Western was built before such technology existed.

Paddington station is likely to be closed for the rest of the week.

Emergency numbers for worried relatives and friends are 0171 834 7777.

All people on the train or witnesses nearby are also asked to call the British Transport Police answerphone on 0800 405040 and leave their name and numbers so they can be contacted as part of the inquiry.

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