London Heathrow, one of the world's busiest airports, suffered massive congestion as many airlines cancelled short-haul flights to Britain on Thursday.

Police said they had arrested 21 people and foiled a plot to blow up planes flying from Britain to the United States, and put the country on its highest state of alert for terror attacks.

They said said that the plot involved hiding explosive devices in hand baggage.

Stringent security measures have been imposed across the country's airports, including requiring passengers with infants to taste bottles of baby milk to prove it did not contain any harmful substances.

Congestion built up rapidly as aviation authorities enforced the new regulations.

Cancellations

British Airways cancelled all its short-haul flights due on Thursday between Heathrow and other British and European cities, as well as the Libyan capital of Tripoli.

Flights across Europe have
been grounded

BA added that it was also cancelling some of Thursday's domestic and shorthaul services in and out of London's second airport, Gatwick.

Airports operator BAA had earlier requested the suspension of all flights due to take off from Heathrow, Europe's busiest  airport.

The inter-connected nature of international air travel meant the disruption spread across the world.

Lufthansa of Germany said it was pulling all flights to Britain  until 1500 GMT. It said 22 flights and 3,041 passengers would be affected, but that it expected flights later in the day to go ahead as planned.

Spain's Iberia said its service to Heathrow would be halted until at least lunchtime.

Train switch

Alitalia grounded all its flights to Britain and aviation authorities in the Netherlands said they were cancelling all flights to Heathrow until at least 1500 GMT.

Security has been ramped up
at airports 

French airline Air France said it had cancelled its three morning flights from Paris to London due to the situation.

Passengers who had already boarded an Air France flight to London were told to disembark from their plane on the tarmac at Paris' Charles de Gaulle Airport.

The chaos resulted in 2,500 extra bookings for cross-channel Eurostar train services from London, Paris and Brussels.

In Asia, airlines said they were waiting for further information  and that it was highly possible they would have to cancel flights to Britain.

Meanwhile Michael Chertoff, the US homeland security secretary, said the threat level for commercial flights from Britain to the US has been raised to "severe", or red.

US security

US airlines said they were not planning to cancel flights to Britain but warned of severe delays.

All US carriers flying from Britain informed passengers of strict new limitations on carry-on baggage in line with emergency directives issued by the British government.

"It's a fluid situation. But we have no cancellations of UK-bound flights," said Brandon Borrman, a spokesman for United  Airlines.

New security measures have
caused mass congestion

"We are experiencing delays of at least two hours out of  (London) Heathrow," he added.

Delta noted that additional security measures had been put in  place for all flights bound for Britain.

"Delta continues to operate its flights from the UK to the US,  although customers may experience delays due to revised check-in requirements issued by the UK's Department for Transport," it said in a statement.

All the major US carriers including American Airlines and  Continental warned passengers not to bring any fluids or gels such  as toothpaste on to flights departing from British airports.