The copycat attacks on London suggest that the terrorists behind the July 7 blasts are still at large and intent on causing havoc and bloodshed, according to terrorism analysts.

Robert Ayers, a security analyst at Chatham House in London, said that that he believed that the same group was behind both attacks.

"All along I've been saying that you had four guys that died [in the July 7 bombings], but the infrastructure that trained them, equipped them, funded them, pointed them at the right target - the infrastructure’s still in place, still here," he told the Reuters news agency.

But if the same group was involved, the obvious question was why the first wave of attacks was so professional and deadly, and second was apparently so amateur, continued Mr Ayers, a former US intelligence official.

He pointed out that police had recovered unused explosives from various sites, including a hire car abandoned by bombers at Luton. Police carried out ten controlled explosions on the hire car in Luton station car park before they placed it on a low-loader and took it away.

"One speculation I’ve had all along is that they left those explosives in the car for another group to pick up and carry out a second attack, but when they got there the car had already been taken over by the police, so they have had to cobble something together fairly quickly," he said.

"From what I’ve been able to gather, either the bombs themselves are very, very small compared to two weeks ago, or they’ve got a manufacturing problem and only the detonators are going off, and not the primary charge. They’re certainly using explosives that aren’t nearly as powerful."

Experts agreed that there were two explanations for today's attack. The first, more benign, is that the attacks were carried out by "imitative amateurs" inspired by the July 7 blasts. The second, more worrying, was that the same group behind the suspected al-Qaeda linked attackers had struck again.

That would show that, far from exhausting its strike potential, the group was capable of causing fresh havoc despite heightened security precautions and a high state of alert among both the police and public. It also would show that the group could readily mobilise fresh operatives - perhaps even would-be suicide bombers - to follow the example of the four bombers who blew themselves up.

Michael Clarke, a security expert at King’s College London, told Reuters, said: "The more we know about the bomb attack two weeks ago, the more skilful it looks, well planned - the people behind it know what they’re doing."

Prof Paul Rogers, of Bradford University, agreed that the second wave of attacks was an "ominous" development. He said: "It implies there might be another cell primed and ready to attack. The one ominous thing is that this appears to be a group of a similar nature to the previous July 7 bombers."

Prof Rogers said, however, that the apparent failure of the devices to detonate on the Underground lines would provide investigating teams with crucial evidence for the earlier attacks.

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