A plot by a British-based al-Qa'ida cell to blow up 10 transatlantic airliners has been foiled, counter-terrorist officers say.
The plan to commit what one senior police officer described as "mass murder on an unimaginable scale" was within hours of being activated, it was claimed.
Twenty-four suspected terrorists were arrested in a series of raids in south-east England and Birmingham following the biggest surveillance operation undertaken against alleged al-Qa'ida followers.
Stringent security measures were imposed at all UK airports yesterday, causing widespread chaos and the cancellation of hundreds of flights. The national security alert was also raised to the highest level - "critical" - for the first time.
The terrorist cell is accused of planning to smuggle a liquid explosive hidden in soft drinks bottles on to aircraft bound for the United States and using a battery to detonate it while on board.
The suicide bombers allegedly intended to carry out three "phased" attacks on nine or 10 jets over a period of several months. The plan, it is understood, was to blow up the aircraft over the sea so that investigators would be unable to discover how the explosive - possibly a peroxide-based liquid explosive - was taken through the airport security without being detected. A counter-terrorist source said: "This had the potential to be bigger than the 9/11 attacks [on the US]."
A joint investigation by police and MI5 reached a critical point on Wednesday night after it emerged that the plotters might be ready to strike within 48 hours and anti-terror officers swooped in the early hours, arresting 24 young Britons, mostly of Pakistani origin, in London, Buckinghamshire and Birmingham.
One unconfirmed report from the US said that two "martyrdom" videotapes were discovered by police during their searches. Another report claimed that five suspects were still being hunted.
John Reid, the Home Secretary, said that if the plot had succeeded, it would have caused death on an "unprecedented scale".
In the US, President George Bush said the plot was "a stark reminder that this nation is at war with Islamic fascists who will use any means to destroy those of us who love freedom".
Michael Chertoff, the US Homeland Security Secretary, who was briefed by the British, confirmed that the plot was in the "final stages" of planning. The US-bound airlines targeted were United, American and Continental, which fly to New York, Washington and California. The US administration raised the threat level for flights from Britain to "red", designating a severe risk of terrorist attacks. It banned beverages, hair gels and lotions from flights, saying only that liquids had been identified as a risk by the investigation in Britain.
The plot is thought to have been partly inspired by a similar plan hatched by an al-Qa'ida terrorist, Khalid Sheikh Mohammed, who intended to detonate bombs on airliners over the Pacific in the 1990s.
Deputy Commissioner Paul Stephenson, of the Metropolitan Police, said: "This was intended to be mass murder on an unimaginable scale."
Airports in Britain introduced a ban on hand luggage and taking liquids on board, which led to massive queues. Parents were being told that if they wished to take baby milk on board they would have to taste it in front of security staff.
Of the 24 suspects arrested so far, sources indicated that the majority, if not all, were British citizens. ABC News in the US reported that two of the alleged ringleaders were said to have travelled to Pakistan and later had money wired to them from Pakistan to buy plane tickets. Several suspects were also arrested in Pakistan as part of the co-ordinated operation, said the Foreign Office.
Searches were still taking place last night at a number of addresses in Walthamstow, east London, High Wycombe, in Buckinghamshire, and Birmingham. Documents and computer equipment have been removed, along with what police describe as "objects of interest". There were no confirmed reports of bombs being discovered.
In High Wycombe, anti-terrorist squad officers raided three separate addresses on the northern outskirts of the town, arresting several men. All the properties were still being searched yesterday, along with a patch of woodland near one of the houses. One of houses, in Walton Drive, belonged to an Asian family called Sawar who had lived there for some years. There were said to be three sons and two daughters, Neighbours said two of the brothers, Amjad and Asad Sawar, were married and lived there with their wives.
A short distance away, police raided a house in Hepplewhite Close, and are believed to have arrested a man named Don Stewart-Whyte, 19 - a white convert to Islam. Mr Stewart-Whyte, a salesman at an electrical store, lives at the house with his mother, who is a schoolteacher, and his wife, who is believed to be of Asian or Arabic background. They married recently after his conversion.
Whitehall sources said the anti-terror operation had gathered pace over the past few days and Tony Blair was fully briefed on its progress before he flew on holiday to Barbados.
At 10pm on Wednesday, John Reid, the Home Secretary, chaired the first meeting of Cobra, the Whitehall civil contingencies committee, which includes security and police chiefs. After a three-and-a-half-hour meeting, at which the police raids were approved, the committee broke off, resuming at 5am.
Douglas Alexander, the Transport Secretary, interrupted his holiday in the Hebridean island of Mull after an official travelled to Scotland to brief him.
Mr Blair was alerted to the crisis and was in constant contact with the Home Secretary. One official said: "What with the situation in the Middle East, the PM has barely been off the phone." One of Mr Blair's first calls was to Mr Bush to brief him on the terror plot and the planned arrests.
Mr Reid took immediate charge of the crisis, marginalising John Prescott, the Deputy Prime Minister, who is nominally in charge in the Government in Mr Blair's absence. The Home Secretary and Mr Alexander called a Westminster press conference to urge public vigilance. Mr Reid said the plot, if it had succeeded, would have caused death on an "unprecedented scale".
Mr Blair said: "I would like to pay tribute to the immense effort made by the police and security services who, for a long period of time, have tracked this situation and been involved in an extraordinary amount of hard work.
"There has been an enormous amount of cooperation with the US authorities which has been of great value and underlines the threat we face and our determination to counter it."
Both David Cameron, the Tory leader, and Sir Menzies Campbell, the Liberal Democrat leader, were briefed on events by Mr Reid.
How events unfolded
Wednesday 9 August
10pm: The Home Secretary, John Reid, chairs a meeting of Cobra, at which it is decided to act to prevent an imminent terrorist attack.
11.50pm: Twenty-one people in London, Birmingham and High Wycombe are arrested in connection with a suspected terrorist plot to blow up aircraft in mid-air.
Thursday 10 August
2am: The Joint Terrorism Analysis Centre raises UK threat status to critical.
5am: First flights scheduled out of Heathrow. Passengers hear they may carry only essential hand luggage.
6.45 am: Reid says police raids are part of "major counter-terrorism operation" against plot designed to "bring down a number of aircraft through mid-flight explosions".
7.30am: Stansted airport asks passengers only to travel if essential.
8.10am: Extra security drafted in at Glasgow, Edinburgh and Aberdeen airports.
8.50am: All incoming flights to Heathrow, not already in the air, suspended.
9.22am: John Reid and the Transport Secretary, Douglas Alexander, deliver "war-style" speeches.
9.30am: British Airways says short-haul UK and European inbound and outbound flights to and from Heathrow are cancelled until 3pm.
9.40am: Tony Douglas, from BAA, says the overnight operation has come as "a surprise" to Heathrow.
10.30am: Deputy Commissioner Paul Stephenson, of Scotland Yard, says: "Aim was to smuggle explosives on to aeroplanes in hand luggage."
11.45am: British Airways cancels 200 flights. Ryanair cancels almost 100.
12.10pm: Head of Scotland Yard's anti-terrorist branch, Peter Clarke, says investigation reached "critical point" on Wednesday night.
12.20pm: Reid says police are confident "main players" are "accounted for".
1pm: Labour MP Mohammad Sawar says demands for a recall of Parliament are intensifying in the wake of the alleged plot.
1.10pm: All easyJet flights out of Gatwick cancelled.
1.30pm: Washington says terrorists targeted three US airlines - United, American and Continental - hoping to hit flights to New York, Washington and California.
2.20pm: US Homeland Security Secretary, Michael Chertoff, describes plot as "well advanced".
3pm: Tony Blair thanks police and security services.
4.20pm: John Prescott arrives at No 10 to attend Cobra meeting.
4.50pm: George Bush thanks Mr Blair for "busting the plot".
5pm: Markets fail to recover. BA shares 5 per cent down, easyJet down 3 per cent.
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