|PIC1|Heathrow Airport has been closed to all incoming flights that are not already in the air, following a police anti-terror operation. A plot has been uncovered to blow up planes in flight from the UK to the US say Scotland Yard.

Terrorists, aiming to commit “mass murder on an unimaginable scale” have been disrupted, according to Scotland Yard. They had planned to detonate explosive devices smuggled in hand luggage on to as many as 10 aircraft.

Police were searching premises with 21 people in custody after arrests in the London area and West Midlands.

The threat is causing delays at all UK airports, with the terrorist threat level to the UK being raised by MI5 to critical. According to MI5's website, “critical threat level”, which is the highest level of threat, means “an attack is expected imminently and indicates an extremely high level of threat to the UK”.

As a result, Heathrow Airport has been closed to all incoming flights that are not already in the air, while several outbound services have been cancelled.

|PIC2|The airport is crammed with thousands of passengers, while it is reported that at Stansted Airport there are more than 2,000 people queuing to pass through customs.

Airport operator BAA asked that all flights still on the ground should not take off for the west London airport. National Air Traffic Services (Nats), which runs air traffic control, said that flights already heading for Heathrow would be allowed to land safely.

A Nats spokesman said, “BAA has asked that no flights due to come to Heathrow be dispatched.”

Metropolitan Police Deputy Assistant Commissioner Paul Stephenson said "We are confident that we have disrupted a plan by terrorists to cause untold death and destruction and to commit, quite frankly, mass murder," he said.

"We believe that the terrorists' aim was to smuggle explosives on to aeroplanes in hand luggage and to detonate these in flight. We also believe that the intended targets were flights from the United Kingdom to the United States of America.

"I can confirm that a significant number of people are currently in custody and the operation is ongoing."

The majority of the arrests were in London with some in the Thames Valley and Birmingham, he said.

|PIC3|Police had spoken to a "good number of community leaders to make them aware that a major operation was under way," he added.

A spokesman for Number 10 said Tony Blair had briefed US President George Bush on the situation during the night, according to the BBC.

The Department for Transport set out the details of the security measures at UK airports.

Passengers are not allowed to take any hand luggage on to any flights in the UK, the department said.

Only the barest essentials - including passports and wallets - will be allowed to be carried on board in transparent plastic bags.

"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," the statement said.