Traces of military plastic explosive, more deadly and efficient than commercial varieties, are understood to have been found in the debris of the wrecked Underground carriages and the bus.

Determining the origin of the explosives is vital and, as The Times has disclosed, one man is believed to have assembled all four devices. Scotland Yard has asked its counterparts around Europe to check stockpiles at military bases and building sites for missing explosives.

Military explosive is hard to detect, easy to hide, stable and, if smuggled across a European border and then into Britain in a drum or other container, would most likely evade any explosive-sniffing devices.

The availability of Semtex, originating from a Czech factory and used extensively by the Provisional IRA, has dried up as a result of intensive efforts. However, there are a number of alternatives, notably C4, which comes in sticks and can then be moulded into a shape suitable for a bomb. Military sources said that 10lb of C4 — the size of each of the London bombs — would fit into a shoebox or standard rucksack.

C4 is a high-quality plastic explosive that has been used by al-Qaeda-affiliated terrorists in other attacks. Richard Reid, the British shoe bomber, hid ten ounces in each of his shoes when he boarded Flight 63 in Paris on December 22, 2001. Indonesian police found traces of C4 at the Bali bomb scene in October 2002; and C4 was used by the terrorists who attacked the American warship, USS Cole, in Yemen in 2000.

The explosive is manufactured mainly in the US but there has been evidence that military explosives have been bought by terrorist groups from sources in Croatia and elsewhere in the Balkans, a region heavily imbued with criminal organisations.

Islamic militants are reported to have obtained military explosives from sources in Belgrade in recent years.

Forensic scientists have told The Times that the construction of the four devices detonated in London was very technically advanced. “You keep hearing that terrorists can easily make a bomb from using instructions on the internet. You can, but not of the design and sophistication of these devices. These were well put together, and it would appear the bomb-maker has highly developed skill,” one expert said.

The trigger device was “almost identical” to the ones found in the rucksack bombs used in the Madrid bombings in March last year — although the terrorists used industrial dynamite stolen from a quarry in northern Spain rather than plastic explosives.

Investigators have not determined whether the London bombs were set off by synchronised alarms using mobile telephones — as they were in Madrid — or some other device such as a watch alarm.

Superintendent Christophe Chaboud, head of the French security service’s Anti-terrorist Co-ordination Unit, said: “The use of military explosives is very worrying. We are more used to seeing home-made explosives made from chemicals.

“How did they procure them? Either they were supplied by the underground market, for example from the Balkans, or they benefited from accomplices who removed explosives from a military base.”

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