The United States reacted quickly today to news that a major terrorist plot had been foiled by British security forces, slapping the highest terrorism alert on commercial flights from the UK and tightening security precautions for all departures from US airports.

Michael Chertoff, who heads the Homeland Security Department set up after the terror attacks of 9/11, told an early morning press conference that the alleged plot in the UK was "suggestive of an al-Qaeda plot". He added: "We were really getting quite close to the execution phase."

Mr Chertoff and other US security chiefs had been kept well briefed by their British counterparts both before and after the overnight swoop in which 21 suspects, believed to be British citizens of Pakistani origin, were arrested.

Those contacts included a telephone call last night between Tony Blair, who has joined his family on holiday in Barbados, and President Bush, on vacation at his Texas ranch.

US officials even supplied some extra details about the plot, said to involve simultaneous mid-air explosions on multiple commercial aircraft cross the Atlantic from the UK.

Counter-terrorist experts quoted by the Associated Press named three of the airlines targeted as Continental, United and American. The flights in question were thought to be travelling to New York, Washington and California.

They also explicitly confirmed what was only hinted at by British officials - that the explosives could involve liquids that were innocuous on their own but deadly when mixed.

"Certainly one of the considerations or concerns that we’ve had is the possibility of bringing on board a number of different components of a bomb, each one of which would be benign but when mixed together would create a bomb," Mr Chertoff said.

He added: "Some of the threats which led to this investigation had been pursued by British authorities for some considerable period of time. However it is only recently, certainly within the last two weeks, maybe less, that the investigation revealed that this planning was taking the direction of targeting the United States.

"This is not a circumstance where you have a handful of people sitting around coming up with dreamy ideas about terrorist plots. The conception, the large number of people involved, the sophisticated design of the devices that were being considered and the sophisticated nature of the plan, all suggest that this group that came together to conspire was very determined, and very skilled, and very capable."

Mr Chertoff said there was nothing to suggest the target date was September 11, the fifth anniversary of 9/11. "Nor can I tell you that they would have waited that long," he added.

He said that the plot was reminiscent of one hatched by the 9/11 mastermind Khalid Sheikh Mohammed in the 1990s to detonate bombs on airliners travelling over the Pacific Ocean.

Asked about the threat to airliners travelling from the UK to the US now, he said: "There’s sufficient uncertainty about whether the British have scooped up everybody that we do think it’s prudent to regard this particular route as still being at the highest level of risk."

The US administration raised the threat level for flights from Britain to "red", designating a severe risk of terrorist attacks - the first time that it had raised an airline threat status to the highest level. All other flights, including all domestic flights, were put under an "orange" alert - one step below.

Heightened security caused long lines and delays at airport security checkpoints. The government banned passengers from carrying all liquids and gels, including toothpaste, makeup, suntan lotion. Baby formula and medicines were exempted.

At Dulles International airport outside Washington, Homeland Security put up hastily printed signs warning passengers in all-red capital letters: "No liquid or gels permitted beyond security."

The signs were taped up at ticket counters as well as the security checkpoints. Passengers dumped liquids into large trash cans that were being emptied every couple minutes. People threw away water bottles, juice boxes, makeup and even a bottle of tequila.

Catherine Philp, a Times correspondent in Washington, said from the Dulles airport that passengers were being allowed to take on hand baggage - unlike in Britain where only the bare essentials were being allowed aboard.

Queues were up to 300 metres long and some passengers, who had not expected the extra security, had missed their flights. "There's no sign of panic or anything," she said. "Nobody seems overly fearful."

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