The thesis is they were going to blow the planes mid flight, over the ocean so the evidence would be lost.
UPDATE FROM TIMES OF LONDON; MOSTLY PAKIS; NINE PLANES TARGETED
'We have foiled mass murder on an unimaginable scale'
By Jenny Booth and Stewart Tendler, Crime Correspondent
Plot to blow up aircraft foiled
21 arrests in London, Birmingham
In-bound flights to Heathrow stopped
No hand luggage on aircraft
What you can take on board
Latest travel news here
A major terrorist plot to commit mass murder on an unimaginable scale by exploding up to nine aircraft in mid-flight between Britain and America has been thwarted, Scotland Yard announced this morning.
So far 21 suspects - believed to be British citizens, many of Pakistani origin - have been arrested in overnight raids in London, the Thames Valley and Birmingham, but police stressed that the huge, complex, security operation was still ongoing and that there may be further arrests.
Both Britain and America have raised their terrorism security alert to the highest level.
Meanwhile scenes of chaos were unfolding at British airports today as draconian security measures were put in place to prevent explosives being carried on board in hand luggage.
Queues of holiday and business passengers snaked outside airport terminals, as many flights were cancelled and Heathrow announced that it was closing to all incoming flights that had not already taken off. Numerous European airlines announced they would not be flying to UK. The disruption is expected to last several days.
Deputy Commissioner Paul Stephenson said in a briefing today: "We are confident that we have disrupted a plan by terrorists to cause untold death and destruction and to commit mass murder.
"We believe that the terrorists' aim was to smuggle explosives onto aeroplanes in hand luggage and to detonate these in mid-flight. We believe the target was flights between the United Kingdom and the United States of America.
"There are 21 people who remain in custody, but the operation is ongoing. We believe we have been very successful in arresting suspects, but this is a very early stage of a very extensive and complex operation. It is a very, very serious plot... Put simply, this was a plot to commit mass murder on an unimaginable scale."
Senior police sources told The Times that they believed up to nine flights to the US were going to be attacked simultaneously, probably later this summer. The suspects in custody are believed all to be British citizens, many of them of Pakistani origin.
Detectives are currently searching a number of addresses. Meanwhile police leaders have spoken to community leaders to keep them in touch with the investigation. Mindful of the outrage amongst the Muslim community when Met anti-terror officers raided a house in Forest Gate last month, Mr Stephenson was careful to stress that Muslims were not being targeted by the police.
"This is not about communities: it is about criminals, murderers, people who want to commit mass murder. This is about people who might masquerade in the community, hiding behind certain faiths, but who want to commit acts that no right-minder person would want to applaud," he said.
Airports banned all hand baggage on board planes except essential travel documents, money and prescription medicines, carried in clear plastic bags.
All passengers were being body-searched, their shoes X-rayed and their pushchairs, walking sticks and belongings were being screened. Only airport wheelchairs were being allowed past security. Passengers for the US were being searched again at their boarding gate.
As the number of cancelled flights began to mount, passengers who had already checked in their suitcases containing all their personal belongings - including their car keys and phones - were facing hours more delays in retrieving their bags so that they could make their way home. Others, in transit from other countries, were hunting for hotels with onward travel very unlikely today.
In a pre-recorded statement, broadcast early this morning, the Home Secretary said that police believed the alleged plot was "very significant indeed".
He said: "Overnight the police, with the full knowledge of ministers, have carried out a major counter-terrorism operation to disrupt what we believe to be a major threat to the UK and international partners.
"The police, acting with the Security Service MI5, are investigating an alleged plot to bring down a number of aircraft through mid-flight explosions, causing a considerable loss of life.
"The police believe the alleged plot was a very significant one indeed. At 2am this morning the Joint Terrorism Analysis Centre raised the UK threat state to its highest level - critical. This is now being publicly announced as I promised to Parliament last month.
"This is a precautionary measure. We are doing everything possible to disrupt any further terrorist activity. This will mean major disruption at all UK airports from today. But as far as is possible we want people to go about their business as normal."
Among the extra security measures announced by the Department of Transport was a ban on carrying any liquids on board, which prompted speculation that the plot might have involved either arson aboard a plane, or mixing two inert liquids to make an explosive chemical compound.
A Department spokesman added: "Regrettably, significant delays at airports are inevitable. Passengers are being asked to allow themselves plenty of extra time and to ensure that other than the few permitted items listed above, all their belongings are placed in their hold baggage and checked in.
"These additional security measures will make travel more difficult for passengers, particularly at such a busy time of the year. But they are necessary and will continue to keep flights from UK airports properly secure.
"We hope that these measures, which are being kept under review by the Government, will need to be in place for a limited period only. In light of the threat to aviation and the need to respond to it, we are asking the travelling public to be patient and understanding and to cooperate fully with airport security staff and the police.
"If passengers have any questions on their travel arrangements or security in place at airports they should contact their airline or carrier."
British Airways said any of its passengers who failed to comply with the Government’s restrictions on luggage and other items would not be allowed on its planes.
David Hill, a former Deputy Chief Constable and police liaison officer on Cobra, the Cabinet's anti-terror emergency committee, said that Cabinet ministers and top police officers would be briefed, and on alert in case a meeting of Cobra needed to be called.
He warned that it was likely that the draconian security measures would remain in place for weeks, to deter any conspirators involved in sub-plots to the main terrorist plot which had been thwarted.
The fact that European airlines had cancelled flights might mean that the plot also had a European dimension, with other plotters based abroad - although it might also be intended to help the British security services by reducing the level of chaos at airports.
"It is the height of the holiday season, and I think that is deliberate," said Mr Hill, who is now a security consultant with the red24 personal security firm. "Terrorists are committed, dedicated killers with not an ounce of compassion for anyone - man, woman or child.
"Plastic explosive can be moulded to look like a chocolate bar, and set off inside a confined, pressurised cabin that quantity would be enough to decimate a plane, wreck it, virtually obliterate it. A fire aboard a plane at 20,000ft would leave passengers with very little chance of survival.
"If we are to deter suicide terrorists who are continuing to plot while we speak, I think these security measures will be with us not for just hours or days, but for quite some time."
Extra security measures at airport:
Passengers may take through the airport security search point, in a single (ideally transparent) plastic carrier bag, only small personal and essential items, such as keys, wallets and nappies (full list below).
Nothing may be carried in pockets.
All passengers must be hand searched, and their footwear and all the items they are carrying must be X-ray screened.
Pushchairs and walking aids must be X-ray screened, and only airport-provided wheelchairs may pass through the screening point.
In addition, all passengers boarding flights to the USA and all the items they are carrying, including those acquired after the central screening point, must be subjected to a secondary search at the boarding gate.
Any liquids discovered must be taken from the passenger. There are no changes to current hold baggage security measures.
Permitted items as hand luggage:
Pocket size wallets and pocket size purses plus contents (for example money, credit cards, identity cards etc (not handbags);
Travel documents essential for the journey (for example passports and travel tickets);
Prescription medicines and medical items sufficient and essential for the flight (eg, diabetic kit), except in liquid form unless verified as authentic;
Spectacles and sunglasses, without cases;
Contact lens holders, without bottles of solution;
For those travelling with an infant: baby food, milk (the contents of each bottle must be tasted by the accompanying passenger) and sanitary items sufficient and essential for the flight (nappies, wipes, creams and nappy disposal bags);
Female sanitary items sufficient and essential for the flight, if unboxed (eg tampons, pads, towels and wipes);
Tissues (unboxed) and/or handkerchiefs;
Keys (but no electrical key fobs).
I heard that theory also about midflight over ocean, but al Qaeda would want spectacular visuals, so I'm not buying that one even if MI5 currently believes it or knows better and is obfuscating what they know.