David Cameron chairs an emergency Cobra meeting as the aviation industry reassesses its security system for cargo shipments following a failed airline bomb plot on Friday.

Cameron chairs Cobra over cargo plane bomb plot (Getty)

The Prime Minister chaired the meeting in central London as air cargo security comes under scrutiny.

Home Secretary Theresa May is answering questions from MPs about the terror plot this afternoon.

Travellers will continue to have stringent security checks too after it emerged over the weekend that one of the cargo plane bombs had been transported on passenger aircraft before it was discovered.

There are concerns that more explosive devices may still be at large, raising questions about freight transportation security and the difficulty in detecting explosive materials.

A German security official has reportedly said the mail bombs intercepted last week contained 10.58 ounces (300 grams) and 15.11 ounces (400 grams) of the explosive PETN.

The official said that if the bombs had gone off "the explosive effect would have been significant."

One of the packages transited through Germany's Cologne airport before being intercepted at its next stop in England. Another package was intercepted in Dubai.

John Brennan, US deputy national security adviser said: "We're trying to get a better handle on what else may be out there.

"We're trying to understand better what we may be facing.

"It would be very imprudent ... to presume that there are no others (packages) out there."

The terror plot came two years after Lord Carlile warned in his annual review of terror legislation that he was concerned terrorists could use cargo planes as vehicles for bombs.

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'Control orders will have to be continued'

Lord Carlile, the government's independent reviewer of anti-terrorist legislation, told Channel 4 News the UK cannot rely on "good practices being followed elsewhere" on the security of parcels coming in and out of the country.

He said Yemen was "an increasing problem". "In the counter-terrorism review which will be published shortly I trust that the government will reflect the objective evidence which is available as to how one should deal with terrorism suspects who cannot be prosecuted," he said. "This may well mean that for a very limited number of people control orders will have to be continued subject, of course, to detailed scrutiny by the courts.

"(Islamic cleric hiding in Yemen, Anwar Al-Awlaki) is a warning to us all. Al-Awlaki apparently was born or at least brought up in the United States. He's an American Muslim, and there are many fine American Muslims. However, he has become radicalised, he's left the United States and has become the architect of some potentially extremely serious terrorism outrages.

"Exactly the same can happen in this country or in any other country in Europe. We have to be ever vigilant about that."

Dr John Gearson, who studies terrorist groups in the Arabian peninsula, said al-Qaeda in the Arabian Peninsula " has great potential, but it hasn't yet demonstrated it's ability to do much more than be a very effective mouthpiece."

"I don't think it's a conspiracy theory to ask the question 'what does al-Qaeda in the Arabian Peninsula gain from this sort of attack'," he said. "It is a regionally focused body that wants to bring down the Saudi and Yemeni government and take control in those areas.

"But it is staffed and led by people who are very close to Bin Laden. So, yes, in dragging attention to the Arabian Peninsula from Afghanistan and Pakistan you are also meeting the objectives of al-Qaeda centre."

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Explosives travelled on passenger planes

Two devices, hidden in printer cartridges and containing the powerful explosive pentaerythritol trinitrate (PETN), were found on cargo planes at East Midlands airport and in Dubai on Friday. Both packages were addressed to synagogues in Chicago.

Qatar Airways confirmed yesterday that the package intercepted in Dubai was carried on two passenger planes - an Airbus A320 from the Yemen capital Sana'a to Doha, and then another passenger plane from Doha to Dubai.

Both devices were previously believed to have travelled on cargo planes.

A statement on Qatar Airways website said: "Qatar Airways can confirm that a recent courier consignment was carried aboard one of its aircraft from Sana'a to Dubai via Doha International Airport.

"The carrier stated that, as per Chicago Convention, it is not the responsibility of the country in which the cargo transits to x-ray or inspect the cargo. This responsibility belongs to the country from where the consignment originates.

"Furthermore, the explosives discovered were of a sophisticated nature whereby they could not be detected by x-ray screening or trained sniffer dogs. The explosives were only discovered after an intelligence tip off."

'Good manifest information can save lives'

In his annual review of terrorism legislation in 2008 Lord Carlile acknowledged that there were real concerns about the threat of terror attacks via small aircraft and cargo planes.

He warned: "I remain firmly of the opinion that the terrorist traveller has at least as great a prospect of being caught at UK ports of entry as anywhere else. I am less confident about cargo, including parcels as stated above.

"The potential use of small aircraft as vehicle bombs against places of public aggregation is a risk that must be guarded against. This is not founded on any particular intelligence, or on any operation as such. However, I know that some knowledgeable police officers and officials have ongoing concerns about the relative simplicity of terrorism conducted in this way, given the very large number of private aircraft and small airfields.

"This has led to ever-developing local policing plans, involving special branch and other police officers working together and with local communities. There is real co-operation from pilots of all kinds of aircraft and owners/operators of air fields of all sizes. I am pleased to report that in every police area now there are designated officers and others engaged on policing smaller aviation, with the capacity to share information and keep each other informed of concerns. Specific training courses are organised methodically. These are encouraging developments, which are making the country safer.

"It is part of my annual litany to repeat in connection with aircraft and passenger shipping that manifests are a cause for concern. As has been said by me and previous reviewers again and again, the information provided by shippers and carriers is of great value to port officers.

"If police know who is on board an aircraft or vessel, or what is being carried, their knowledge is increased, and they may be able to further important enquiries. If the manifest information is inaccurate, inadequate and given a low level of importance by transport operators, a vital clue may be missed. Good manifest information can save lives, and minimise delays."

The Metropolitan Police responded to criticism the UK device was declared "safe" at first, before being re-examined as a "precaution".

A spokesman said: "Following initial examination explosives officers were satisfied that the package presented no immediate danger.

"It was secured for further examination and cordons were removed.

"Further examination of the package by explosives officers identified items of concern.

"Cordons were put back in place as a precaution prior to items being removed for further scientific analysis."

The terror alert followed a week where airline bosses called for a scrapping of some existing security procedures. But this request is likely to be ignored following Friday's incident.

Yemen under the spotlight

Both devices originated in Yemen and were only found after a tip-off from Saudi intelligence.

An international conference is taking place today in central London to address the Yemen issue. International Development Minister Alan Duncan will give a keynote speech and focus on the challenges that face that country and the UK.

Al-Qaeda Coming Home to Yemen

It seems inevitable that the focus of western intelligence agencies will have to widen and shift from the Pakistan-Afghanistan border back to the Arabian Peninsula, where al-Qaeda began.

So how do we help make Yemen safer, beyond banning flights from its territory and strengthening cargo transport security around the world?

Those who know the country well insist that nothing less than a Marshall Plan of economic development, funded by the rich Gulf States and others, will put the country on its feet.

American boots on the ground is not an option. The occupation by "infidel" forces from the holy lands of Mecca and Medina on the Arabian Peninsula was given as the justification for September 11th, and more Americans would only boost al-Qaeda's cause.

The most likely short term scenario is an increase in unmanned drone attacks on suspected al-Qaeda targets. But bear this in mind - the strength of al-Qaeda in Yemen may be no more than 500 to 700 men. Get the strategy wrong, with inaccurate bombing raids and clumsy Yemeni-led offensives, and the al-Qaeda corps could double in numbers in a matter of months. In short, Britain's chief role in Yemen is to ensure that America does not make a bad situation worse.

Read more

Yemen, the ancestral home of Osama Bin Laden, has become a key front in the fight against terrorism in the past year.

The terror investigation is now focusing on Saudi-born bomb-maker Ibrahim Hassan al-Asiri.

It is believed he is responsible for making the bombs and was also the alleged bomb maker in the failed Christmas Day bomb plot over Detroit last year.

Student, Hanan al-Samawi, 22, was arrested by Yemeni police on suspicion of sending the packages, but a shipping agent claimed she was not the same person who signed the shipping documents.