Red alert after police 'foil air terror plot'

* 'Liquid chemical devices in passenger bags' suspected <br/>* US flights from UK were targeted, say Washington sources<br/>* 21 arrests: main players 'accounted for' says Reid <br/>* MI5 raises the threat level to critical - attack imminent

A terrorist plot to blow up aircraft in mid-flight has been thwarted, Scotland Yard said. Hand luggage has been banned at UK airports and MI5 has raised the threat level to "critical".

A statement on the MI5 website said: "This means that an attack is expected imminently and indicates an extremely high level of threat to the UK."

Sources say that liquid chemical devices were suspected. It is believed that the aim was to smuggle the devices on board the aircraft in hand luggage and that the attacks would have been targeted at flights from the UK to the USA. In Washington, counter-terrorism officials said United, American, Continental airlines were targeted.

Senior sources in London said that the terrorists were believed to have been planning to blow up as many as nine aircraft. The sources could not give an accurate estimate about when exactly the simultaneous attacks were planned for, but said there were "operational reasons" which meant action was required last night.

Scotland Yard has arrested 21 people in London, the Thames Valley area and Birmingham.

Deputy Commissioner Paul Stephenson said that the plot was intended to bring down a number of aircraft through mid-flight explosions. He said: "This was intended to be mass murder on an unimaginable scale."

Searches were on-going at a number of addresses, he added. Sources said detectives had found "a number of things that are causing interest" .

It is believed the covert investigation has been going on for several months and it is understood that officers made the arrests overnight not because they feared an attack was likely to happen today but for other intelligence reasons which meant they had to act quickly.

The head of Scotland Yard's anti-terrorist branch, Peter Clarke, said the plot has "a global dimension". The investigation had involved an "unprecedented" level of surveillance and had involved police forces in the UK and internationally.

The Home Secretary John Reid said today that police had carried out "a major counter-terrorism operation to disrupt what we believe to be a major threat to the UK and international partners".

He said the main players were "accounted for", but stressed the need to maintain vigilant.

Mr Reid said the alleged plot was "very significant" and was designed to "bring down a number of aircraft through mid-flight explosions, causing a considerable loss of life".

He warned that the operation would mean major disruption at all UK airports from today but he added: "As far as is possible we want people to go about their business as normal."

Additional security measures have been put in place for all flights from UK airports.

A Department of Transport spokesman said: "Regrettably, significant delays at airports are inevitable. Passengers are being asked to allow themselves plenty of extra time and to ensure that other than a few permitted items, 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."

A Metropolitan Police spokesman said: "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."