The near-simultaneous targeting of three Tube trains, at Shepherds Bush, the Oval and Warren Street, and later a London bus in Hackney, bore striking similarities to the attacks two weeks ago, which killed 56 people. A police source said that the attacks had been made by a second squad of suicide bombers and examination of the explosives used established a clear link.

Detectives have established from witness accounts that at least two of the men who carried out the attacks were astonished to be alive after the detonations systems for the explosives failed.

This time the sense of fear and shock was mixed with relief because all four bombs apparently failed to detonate properly and there were not believed to have been any serious casualties. Security chiefs believe that this was because all four bombs were made by the same bomb makers and had the same flaw.

Each of their bombs was smaller and fitted with a homemade detonation device, all of which failed to trigger the full blasts. Scientists are also looking at the possibility that if the explosive did come from the same batch as that used on July 7, its quality may have deteriorated in the past two weeks.

Two people were arrested yesterday but Scotland Yard denied that they had been connected with the attempted bombings.

The bomber at Shepherds Bush was startled when the blast from his device merely blew him on to his back, rather than kill him outright. Passengers who saw the explosion said that he appeared shocked that he was able to walk away.

Police sources said that all four detonators were homemade. They said that the detonator used in the attack at Warren Street station consisted of nails packed around explosive.

Sir Ian Blair, the Metropolitan Police Commissioner, said that the intention of the terrorists must have been to kill, but he added that it was too early to conclude whether the latest attacks were the work of the same network that caused such carnage a fortnight ago.

He added: “There is a resonance here. There were four attacks and there were four attacks before. We do believe that this may represent a significant breakthrough in the sense that there is forensic material at these scenes.”

Counter-terrorist experts believe that the failure of the devices will provide fingerprints, DNA and possible matches to the bombs used on July 7. Closed-circuit television from Underground stations and the streets, witness statements and descriptions provided by passengers will be a major part of identifying and catching the bombers.

Senior police officers accepted that they could have been facing fresh casualties across London. Sir Ian said that detectives had begun assembling details of the would-be bombers and he urged the public to send digital photographs or mobile phone images to a Scotland Yard website.

Police praised three men, believed to be two officers and a member of the public, who tackled the bomber at the Oval, ripping his rucksack from his back and securing vital evidence.

The London transport system once again ground to a halt. Armed police were deployed to University College Hospital in London and a man was arrested at gunpoint outside Downing Street, an incident later said to be unconnected with the attempted blasts.

Tony Blair appealed for calm after a meeting of the Cobra emergency planning team. He said: “We cannot minimise incidents such as these. All I would like to say is this: we know why these things are done, to frighten people and make them anxious and worried.”

Mr Blair said that he was determined that the “evil, bankrupt ideology based on the perversion of Islam” espoused by the terrorists would be overcome. “This is something that has built up over a period of time,” he said. “It will have to be dismantled over a period of time. But I have no doubt at all in the end the values that we represent are the values that will triumph.”

The Association of Chief Police Officers also made radical proposals to allow terrorism suspects to be held for questioning for up to three months rather than the current maximum of 14 days.

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