* Liquid chemical device suspected
* US claims al Qaeda link, compares to 9/11
* Pakistan helped thwart plot, makes arrests
* Police say bombers were just days away from carrying out attacks
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An undercover British agent infiltrated the terrorist group planning to blow up as many as 10 jets leaving Britain for the US in a plot equal to 9/11, CNN is reporting.
The American news channel said that several US government officials had told it that an agent managed to give authorities enough intelligence to thwart the plan.
More than 20 people have been arrested, terror threat levels have been raised to some of their highest levels, and hundreds of flights have been cancelled worldwide.
CNN reported that the plotters had not bought plane tickets but were using the internet to find flights to various cities that had similar departure times.
US intelligence officials believe the plotters had planned "a dry run" in the next couple of days and, if they could get on several flights at the same time, intended to carry out the attack imminently, the
New York Times
reported.
'Mother with baby arrested'
PA news agency reported one of the 24 people arrested is a young mother with a six-month-old baby, according to a Muslim community leader.
PA quoted Imtiaz Qadir, a spokesman for the Waltham Forest Islamic Association, saying one of those arrested in Walthamstow, east London, was a woman in her early 20s who has a baby.
"A young Muslim lady was arrested, and she has a six-month-old child. They have taken the child too, because it needs to be with its mother," Mr Qadir told PA.
Police raided at least five properties in Walthamstow overnight and made a number of arrests. The houses have been cordoned off.
"I know five of the men very well and they are really respectable young Muslim men," Mr Qadir told PA.
"I am totally shocked. I don't believe they've done anything to warrant this.
The arrests foiled a terror plot "intended to be mass murder on an unimaginable scale", said Deputy Police Commissioner Paul Stephenson.
US Homeland Security Michael Chertoff said today the foiled plot would have been a disaster comparable to the September 11 attacks.
If the plot had succeeded "we would have seen a disaster on a scale comparable to 9/11 with hundreds maybe thousands of people being killed," Chertoff told PBS.
Security sources said a "liquid chemical" device was suspected and it is believed the aim was to detonate devices smuggled on to the aircraft in hand luggage.
US network ABC News reported the suspects had planned to conceal liquid or gel explosives inside a modified sports beverage drink container and trigger the device with the flash from a disposable camera.
"The plotters planned to leave the top of the bottle sealed and filled with the original beverage but add a false bottom, filled with a liquid or gel explosive," ABC said. "The terrorists planned to dye the explosive mixture red to match the sports drink sealed in the top half of the container."
Peter Neumann, director of the Centre for Defence Studies at London's King's College university, said a liquid explosive attack of this type is particularly worrying.
"Planes remain vulnerable and in the coming weeks terrorists will be thinking of something else to do that we have no idea about.
"If this had worked out it would have been beyond any doubt the biggest terrorist plot in the UK," he said.
Neumann said the plot appeared similar to a 1995 plan to blow up 11 planes using nitroglycerine mixed in contact lens solution and a battery powered detonator hidden in a shoe.
The ABC News network, citing US sources, also said five additional suspects in the airline bomb plot foiled by British police were still at large and being sought.
British police declined to comment on the report that five suspects remained on the loose.
The attacks would have been particularly aimed at flights from Britain to the US, with unconfirmed media reports saying anywhere from six to 10 US commercial airliners had been targeted.
The United States said it suspected al Qaeda involvement and President George Bush said it was a "stark reminder" that his country was "at war with Islamic fascists".
Pakistan said its intelligence agencies helped thwart the plot and had arrested an unspecified number of people.
'Global dimensions'
Peter Clarke, head of the Metropolitan Police anti-terrorist branch, said: "We have been looking at meetings, movements, travel, spending and the aspirations of a large group of people.
"As is so often the case in these investigations, the alleged plot has global dimensions."
Mr Clarke said the number, destination and timing of the flights that might be targeted remained under investigation.
Heathrow Airport, the busiest in Europe, was closed to most European flights. The block on incoming traffic applied to flights of three hours or less, affecting most of the traffic from Europe.
A Heathrow spokesman said: "We would ask passengers not to come to the airport today unless absolutely necessary."
Congestion built up rapidly at the airport as authorities enforced strict new regulations banning most hand baggage.
Home Secretary John Reid said the alleged plot aimed to "bring down aircraft through mid-flight explosions, causing a considerable loss of life".
Police said arrests were made in London and Birmingham.
The suspects were "homegrown", though it was not immediately clear if they were all British citizens, a police official said, adding that police were working with the South Asian community.
It is believed the investigation has been going on for several months, and it is understood officers made the arrests overnight not because they feared an attack was likely to happen, but for other intelligence reasons which meant they had to act quickly.
'High level of threat'
But a statement on the website of the internal intelligence service MI5 said: "This means an attack is expected imminently and indicates an extremely high level of threat to the UK."
Mr Reid said police carried out "a major counter-terrorism operation to disrupt what we believe to be a major threat to the UK and international partners".
The United States' Homeland Security Secretary, Michael Chertoff, said: "We believe that these arrests have significantly disrupted the threat, but we cannot be sure that the threat has been entirely eliminated or the plot completely thwarted."
Airlines remained in chaos this morning.
Britain raised its security threat level to "critical", which meant it believed an attack was imminent.
The United States also raised its threat level for flights from Britain to "severe", the highest level.
The US also took immediate steps that included barring passengers from carrying liquids, including beverages, hair gels and lotions, on planes. It said travellers in both countries should expect long delays.
The flight upheaval occurred as airports prepared for large numbers of passengers going on summer holidays.
All airlines stopped flights from Brussels to Heathrow. Spain's Iberia airline said it had cancelled four flights to Britain, and Italian carrier Alitalia stopped six.
Authorities believe as many as 50 people were involved in or connected to the plot, a senior US counter-terrorism official said.
The terrorists were said to be targeting the major US carriers United Airlines, American Airlines and Continental Airlines.
The plan "had a footprint to al Qaeda back to it", the official said.
The plan involved passengers hiding masked explosives in carry-on luggage.
Another counter-terrorism official said the investigation, which has been going for months, was "very serious" and that US intelligence had been working closely with the British.
NZ Aviation Security Service general manager Mark Everitt said last night he was "assessing all the intelligence that I'm getting and briefings with certain Government agencies".
"I'm not making any changes to the levels of security applied to passengers getting on board aircraft. I'm assessing that ... hour by hour. We will think about it again in the morning."
The news came amid high international tension over the war in Lebanon and in the week British Prime Minister Tony Blair went on holiday to the Caribbean.
Shares in European airlines fell and British Airways shares opened 3.8 per cent down.
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