Explosive devices found on planes 'very sophisticated'

One of US President Barack Obama's security advisers, John Brennan, said the devices, hidden inside printer toner cartridges, were very difficult to detect and were similar in construction to those used in previous al-Qaida terror attempts.

Police officials in Dubai, where one of the would be bombs was discovered, said the bomb there contained an electric circuit linked to a mobile phone chip as well as the compound lead azide, which can be used in detonators.

A spokesman said: "The parcel was prepared in a professional manner where a closed electrical circuit was connected to a mobile phone SIM card hidden inside the printer.

"This tactic carries the hallmarks of methods used previously by terrorist organisations such as al-Qaida."

Prime Minister David Cameron said: "We believe that the device was designed to go off on the aeroplane. We cannot be sure about the timing when that was mean to take place.

"There is no early evidence it was designed to take place over British soil, but of course we cannot rule that out."

Speaking ahead of a meeting with German chancellor Angela Merkel, he confirmed he had spoken to President Obama, adding: "I have also spoken to President (Ali Abdullah) Saleh of the Yemen making the point that we have to do even more to crack down and cut out the cancer of al-Qaida in Yemen and the Arabian Peninsula.

"We have immediately banned packages coming to or through Britain from the Yemen and we will be looking extremely carefully at any further steps we have to take."

Home Secretary Theresa May said the terrorists behind the cargo bomb plot would not have known exactly where the deadly device was to strike and that the unpredictable routes taken by freight planes meant it was difficult to say even now whether the explosions would have happened over Britain or America.

Asked about the potential cost of tougher restrictions, the Home Secretary insisted that was being taken into account. "We are well aware of the economic aspects of freight travel," she said.

Security experts said that the use of such devices was proof that terrorists were coming up with increasingly innovative ways to launch attacks.

The chief executive of the intelligence agency IntelCenter, Ben Venzke, said: "If this attack is by al-Qaida in the Arabian Peninsula it demonstrates an accelerated ability to design new and innovative ways of conducting IED attacks and a focused effort to execute those attacks on US soil.

"The using of many methods for implementation and bombings is very important because it gives flexibility to operations and the infiltration through barriers.The decision to use one device or another differs according to the importance of the location and the results of the blast. You have weapons that you use at the proper time. This is also subject to the conditions of the targeted place.

"The tight security inside the office of a security official under observation and guard is totally different than an aeroplane that is in the air for six hours.

"It is certain that the conditions in the later situation will be more flexible and do not raise suspicions during implementation."

Terror expert Dr Sally Leivesley said it appeared to be a "sophisticated" device which may have used the powdered toner as a means of evading screening.

She said: "It's a step-jump change in terms of threat to aviation. These devices can be put on board anywhere."

Direct cargo and passenger flights from Yemen to the UK were suspended in January this year after an attempt to blow up a Detroit-bound plane on Christmas Day last year.