Emergency services were still working last night to free bodies from the wreckage of the Piccadilly Line (search) train in a tunnel 100 feet below the surface.

The depth and narrowness of the subway tunnel, which is only a few inches wider than the train, were hampering the recovery operation.

It was also delayed by the stifling heat, the risk from asbestos and fears that the roof of the tunnel would collapse.

The official death toll on the train remained at 21 yesterday, but this was based only on those bodies that had been recovered. Police believe several more bodies may still be lying in the wreckage.

Members of London Underground’s emergency response unit spoke of the carnage they witnessed in the front carriage of the train where the explosion took place.

Many of the most severely injured people either walked or had to be carried half a mile along the tracks to Russell Square station. King’s Cross (search) station was much closer but they could not get back past the wreckage.

One young woman died after being laid out beside the ticket office at Russell Square (search).

More than 800 people were believed to have been packed on the six-carriage train. Many more people were killed by the Piccadilly Line blast than by the two on the Circle Line.

Piccadilly Line trains tend to be more crowded and the narrowness of the tunnels would have concentrated the force of the blast.

London Underground said the Circle Line was likely to reopen within a few days but the Piccadilly Line would remain closed for several weeks.

Officials said they were confident that the tunnel was in no danger of collapsing but there was severe damage to the walls, rails and wiring.

Last night, they were considering detaching four or five carriages and towing them back towards King’s Cross to give emergency workers better access to the front carriage.

Once clear of bodies, the wreckage is likely to be jacked up on to rolling devices known as skates and pulled out of the tunnel. The bomb had been placed in the standing area beside the rear doors. There is extra space beside the doors for suitcases because the Piccadilly Line serves Heathrow. It is often difficult to tell which bag belongs to which passenger.

Andy Hayman, Metropolitan Police assistant commissioner responsible for anti-terrorism, said the wreckage would be cleared only after forensic science officers had completed their work.He said: “There is a risk of the tunnel being unsafe. I ask everyone’s patience as we progress this matter.

“It would be unwise and could inhibit a successful prosecution if we rushed this stage. Just imagine an explosion that far into a tunnel. I think we can all respect the sort of things our people are actually confronting. I think out of respect for others that’s probably the most detail I’d want to go into.”

He said that with the passage of time there would also be the hazard of vermin in the tunnels and “dangerous substances” in the air. Transport for London said later that the concern was not about fumes from damaged equipment but from bodies beginning to decompose in the heat.

A maintenance worker, who did not want to be identified, said he had reached the site early yesterday and seen scenes he described as horrific. He said: “We got up to the carriage, although it was very dark there at the time. The smell was awful.”

Andy Trotter, Deputy Chief Constable of British Transport Police, said “There are no living people in there and the challenge is now to remove the dead.

“This is an enclosed tunnel and it is very difficult conditions and it’s a bit dangerous. It will take some time before everyone is removed from there.”

He confirmed that it had taken hours for some survivors to get off the train. “That was an extremely crowded rush-hour train with several hundred people on board.

“Some people had obviously walked away from the explosion and walked several hundred yards to Russell Square.

“They were streaming out of there and many of them came into our headquarters and other offices in the area. There were others who were more seriously injured. It was a matter of hours before people were removed. It was a difficult and chaotic situation.”