Six planes were involved and flights were targeted from London, Glasgow and Manchester, police said.

    Police have arrested 20 people in London, according to SKY report, and they are still looking for two people. All those arrested are British Muslims, SKY reported.

    Reid said Thursday morning that British MI5 and police had foiled the attempt to detonate explosives devices smuggled aboard as hand luggage on mid-night flights from Britain to the United States.

    "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," he 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," he said.

    The London Heathrow airport is now closed to all incoming flights. British Airways (BA) has cancelled all short-haul European flights as well as severe delays to services. And OlympicAirways has grounded all flights.

    Air France and KLM have cancelled all flights to Britain. And all flights from Brussels to Britain have also been cancelled.

    Responding to the incident, Britain's Joint Terrorism Analysis Centre raised the country's security threat level to "critical" from "severe."

    Britain has a five-level rating for security threats, ranging from low to critical, which is the highest level

    Security at the airports across the country has beefed up, and massive delays have been caused at all airports cross Britain.

    BA on Thursday advised all passengers that they would not be permitted to carry any hand baggage on board any aircraft departing from any airport in Britain.

    Passengers will be allowed to take only travel documents, eyeglasses and a handful of other items through airport security, the company said in a statement.

    BA also advised passengers that no electrical or battery powered items including laptops and mobile phones could be allowed in the cabin.

    Meanwhile, The Downing Street said that Prime Minister Tony Blair, on holiday in the Caribbean, is in constant contact with the UK anti-terror team. The operation undertaken is with full support from the prime minister, it said.

    Blair also kept U.S. President George W. Bush informed on security threat to planes, the Downing Street added. The United States has raised threat level in response to the terror plot in Britain. Enditem

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