British police say they have foiled a major plot to blow up aircraft in mid flight.
More than a dozen people have been arrested in a major counter terrorist operation.
Our correspondent in London, Stephanie Kennedy, reports that police say the aim of the plot was to detonate bombs smuggled on board aircraft in hand luggage.
The alleged plot is believed to have been aimed at flights from Britain to the United States.
The British home secretary John Reid says the plot was "very significant" and designed to "bring down a number of aircraft through mid-flight explosions, causing a considerable loss of life."
Security has been increased at all British airports and major delays are being experienced.
Until further notice, passengers will not be allowed to take hand luggage on to any flights, except for essentials, such as passports.
Essential items will be allowed to be taken on board in transparent plastic bags.
Britain's threat level has been increased from severe to critical, the highest of its five ratings.
A major terrorist plot allegedly to blow up multiple aircraft in mid flight from London to America has been stopped and 21 people arrested, police have said.
"Put simply, this was intended to be mass murder on an unimaginable scale", the Met's deputy commissioner Paul Stephenson said in a strongly-worded statement this morning.
"We believe that the terrorists' aim was to smuggle explosives onto airplanes in hand luggage and to detonate these in flight."
A total of 21 people were arrested overnight - mostly in London, but also in the Thames Valley and Birmingham, police have confirmed. The suspects, thought to be British-born, are being questioned.
continued...
Home secretary John Reid said the plot was believed to be "a very significant one indeed" to "bring down a number of aircraft through mid-flight explosions".
The planned explosions may have been simultaneous on as many as nine aircraft, possibly over the Atlantic as to minimise forensic evidence, according to reports.
Heathrow closed to incoming flights
For the first time ever all UK airports, including the two busiest at Heathrow and Gatwick, have been put on "critical" security alert at 2am this morning. "Critical" alert, the highest warning level, is defined by MI5 as fear of an "imminent" terrorist attack.Heathrow has been closed to all incoming aircraft not already in the air, while all outbound short-haul flights have also been grounded. The knock-on effect on delays means there is no room for the planes to park.
Brussels international airport, the German airline Lufthansa, Air Iberia and the Greece national carrier airline Olympic Airlines have cancelled all flights to Britain.
No hand luggage is being allowed on UK planes, apart from a few exceptions like wallets and keys. These can be carried in a transparent plastic bag.
Any liquids are also banned.
All other items, including laptops, mobile phones, iPods, books and newspapers, have to be searched and checked in along with normal luggage. Baby milk and food can be carried on board, but must be tasted in front of airport staff.
The unprecedented safety measures have already led to substantial delays. Passengers are queuing for more than five hours to go through security.In America, the threat level has also been raised to the maximum of "red". No liquids, including beverages and hair gels, are allowed to be carried onto any flights, whether domestic or international. It is the first red alert in the US triggered by a threat in another country.
"We are confident that we have disrupted a plan by terrorists to cause untold death and destruction and commit mass murder," the Met's Mr Stephenson said.
"We also believe that the intended targets were flights from the UK to the USA."
Two US security officials have been quoted saying the targeted airlines were United, American and Continental.
Attacks 'not today'
This morning's arrests were part of a "pre-planned intelligence led operation" by anti-terrorist police and MI5, the Met said in a statement.
"Today's arrests are the culmination of a major covert counter-terrorist operation lasting several months."
It is not thought the attacks would have gone ahead today, but police chose it at their moment to swoop.
Searches are still ongoing at several addresses, including at least one house in High Wycombe, Buckinghamshire. Streets around Micklefield Road and Forest Way in the west of the town have been cordoned off, with police helicopters hovering overhead.
"This is a major operation, which will inevitably be lengthy and complex," Mr Stephenson added. The UK is now at the highest possible level of alert."
A "good number" of community leaders were told that the raids would take place, but without giving specific details, he said.
In a pre-recorded statement televised early this morning, the home secretary said: "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," Dr Reid added.
"At 2am this morning the Joint Terrorism Analysis Centre raised the UK threat state to its highest level - 'critical'."
"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."
'Fly only if essential'
Passengers were advised to check with their airlines before travelling to airports.
"British Airways is experiencing severe delays to its operations on Thursday, August 10, as a result of the current security situation,'' BA said in a statement.
"It is likely that some flights will be cancelled during the day. Any customers who do not need to travel today are advised to stay at home."
in pilon's direction.
