As body-recovery and forensic teams continued their search of a Tube train near King’s Cross, it emerged that up to

20 bodies might still be trapped in the debris. The official death toll was put yesterday at 49, with more than 700 people injured. Police said it could take another two weeks to identify all those who died at King’s Cross.

Police warned that the bombers could strike again and last night evacuated parts of Birmingham city centre, saying intelligence indicated “a credible threat to public safety”.

Shortly before 9pm more than 20,000 were forced to evacuate pubs, bars and restaurants in the centre, including the Broad Street entertainment district. Four controlled explosions were carried out on a suspect package on a bus, though its contents were later found not to be suspicious.

But thousands were still prevented from returning to their homes early this morning while a bomb disposal squad investigated a second package in the Broad Street area.

Police say the bombs that detonated on three Tube trains last Thursday all exploded within 50 seconds of each other. Detectives from SO13, Scotland Yard’s anti-terrorist branch, are working on the theory that the devices, each comprising less than 10lb of high explosive, were detonated by a timing device set for 8.50am.

They believe the bombs were carried onto the trains by at least three separate terrorists who then left. The bombs appear to have been placed on the floors of the carriages, near the doors. Together with the bus bomber, police say the terror cell comprised at least four individuals.

Detectives say the fact that the Underground bombs, at Edgware Road, Liverpool Street and King’s Cross, all exploded within a minute suggests a level of planning comparable to the train attacks in Madrid, which killed 191 people last year.

The investigation to find the terror cell involves more than 1,000 officers and is probably the biggest mounted in Britain.

Several hundred police officers have been called in to examine footage from thousands of CCTV cameras collected from Tube stations and high streets around London. However, the camera on the bus that exploded at Tavistock Square and which could have identified at least one suspect had not been working since mid-June.

Debris from the four bomb sites has been taken to the forensic explosives laboratory in Fort Halstead, Kent. Scientists there are expected to confirm what type of explosive was used. That in turn could provide vital clues about other bombings and provide evidence about the likely bombmaker.

Police have ordered a huge manhunt because it is feared the bombers have access to more explosives. Deputy Chief Constable Andy Trotter of British Transport police said it was “perfectly possible” that there might be further terrorist attacks.

The nation will fall silent for two minutes at noon on Thursday to mark the week’s anniversary of the attacks. Ministers discussed the idea at a meeting of Cobra, the cabinet emergency committee, yesterday.

Officials are drawing up plans to designate a public park, probably near Tavistock Square or King’s Cross, as a “shrine” for those who want to place flowers or simply pray in remembrance of the victims.

A 24-hour support centre for families of victims of the London terror attacks was set up in the capital yesterday to offer practical and emotional support to people whose relatives have been killed or are missing.

Rowan Williams, the Archbishop of Canterbury, will join Britain’s other main religious leaders today in saying that they stand “shoulder to shoulder” in condemning the attacks.

A commission chaired by the Westminster coroner Dr Paul Knapman has been appointed to oversee identification of the dead. Police said 13 bodies, all from the bus bombing, were lying in a mortuary in Chelsea. Officers hope one of them might be that of a terrorist, whose body could provide clues to the cell behind the attacks.

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