A major terrorist plot to commit 'mass murder on an unimaginable scale' by blowing up aircraft flying from the United Kingdom to the United States has been foiled with the arrest of 21 people in London [Images], Birmingham and Thames Valley in an ongoing operation.

The plot was 'very significant' and was designed to 'bring down a number of aircraft through mid-flight explosions causing a considerable loss of life,' British Home Secretary John Reid said on Thursday.

British Intelligence agency M15 said the current threat level is critical -- the highest possible -- meaning thereby 'an attack is expected and indicates an extremely high level of threat to the UK.' Security sources believe that some liquid explosives could be mixed during the flight into a lethal concoction. Security at all airports in Britain has been increased with additional security measures put in place for all flights.

Heathrow airport, the busiest in Europe, was closed to all incoming flights that were already in the air while several outbound services were cancelled causing inconvenience to thousands of travellers.

"The plot was intended to be mass murder of unimaginable scale," Metropolitan Police Deputy Commissioner Paul Stephenson said, adding: "We believe that the terrorists' aim was to smuggle explosives onto aeroplanes in hand baggage and to detonate these in-flight."

"This is about people who are desperate, people who want to do things that no right-minded citizen of this country or any other country would want to tolerate," he said.

Domestic news agency Press Association quoted unnamed senior police sources as stating that majority of those arrested were believed to be of Pakistani origin but most, if not all, were British.

In the US, the Department of Homeland Security raised the terror threat to the highest level of 'severe' or red for commercial flights originating in the UK and bound for US. It also prohibited any liquids, including any beverages, hair gels and lotions, from being carried on aircraft. In addition, the threat level has been raised to high for all commercial flights operating or coming to the United States, a DHS statement said.

Later in the day, British Prime Minister Tony Blair [Images] discussed the situation with US President George W Bush [Images].

People have been advised to carry only the barest essentials -- including passports and wallets -- on board aircraft in transparent plastic bags. There are no changes to current hold baggage security measures.

British Airways said any of its passengers who failed to comply with the Government's restrictions on luggage and other items will not be allowed on its planes. A spokesman said passengers were advised that no electrical or battery powered items, including laptops, mobile phones, iPods and remote controls, could be carried into the cabin and must be checked in as hold baggage.

"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," a Metropolitan Police spokesman said.

"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," he added.

"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. We would like to reassure the public that this operation was carried out with public safety uppermost in our minds. This is a major operation which inevitably will be lengthy and complex," he said.

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