Deputy Assistant Police Commissioner Andy Trottter said it was possible that a further 100 people were missing.

The BBC's Simon Montague: "ASLEF is threatening a nationwide strike over safety"

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.

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

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

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.

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.

Deputy Assistant Commissioner Andy Trotter, of the Metropolitan Police, said the search for bodies was concentrating on the front first-class carriage of the Great Western train, which was "devastated" by fire.

The grisly search was suspended on Tuesday night with the emergency services sure there was no one else left alive on either train.

Assistant Deputy Commissioner Andy Trotter: "Family liaison is uppermost in our minds at the moment"

Mr Trotter said a 100-ton crane was on its way from Stockport, Greater Manchester, to help lift some of the wreckage.

He said the search would be a "laborious" task and pointed out, with much of the wreckage in a precarious position, the safety of rescue workers was a priority.

There are reports that some of the rescue workers are to be sent home because they have been traumatised.

Of the 27 confirmed dead, 12 bodies remain at the scene but he said he hoped they would all be removed by the end of Wednesday.

Police say the search will be a long and laborious task

Mr Trotter refused to comment on speculation about the final death toll but said there were about 500 people on the Great Western train and at least 100 on the Thames train.

Superintendent Tony Thompson, of the British Transport Police, said four special centres had 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 Railway Station in Reading, Berkshire.

As well as the 27 confirmed dead, 165 were taken to hospital. Forty-seven 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.

He also ordered Health and Safety Executive investigators to issue a preliminary statement on the crash within days, to avoid a delay like that which followed the Southall rail crash.

Seven people died and 150 were injured in the Southall crash in 1997. The public inquiry only began last month, and critics say valuable safety lessons may not have been learned.

Supt Tony Thompson, of British Transport Police: "It's a very laborious task and it's going to take some time"

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.

There have been claims the red signal, number 109, which is the focus of attention over Tuesday's accident, had been passed eight times since 1993.

More than 160 people were injured

Click here to see the site of the crash

Investigators are likely to concentrate on the state of the signals, the possibility of human error and the condition of the track.

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

The BBC's Susanna Reid: "Rail users will need more reassurance"

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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