Bomb plot 'has all the hallmarks of al-Qaida'

He suggested they could have used liquids that were innocuous on their own but could be potentially deadly when mixed.

“This operation is in some respects suggestive of an al-Qaida plot,” Mr Chertoff told a press conference in Washington DC.

He said the plan was “well advanced” and “really quite close to the execution phase“.

“They had accumulated and assembled the capabilities that they needed and they were in the final stages of planning for execution,” Mr Chertoff added.

FBI director Robert Mueller also pointed to a possible al-Qaida link. “This had the earmarks of an al-Qaida plot,” he said.

US air marshals are being sent to the UK to provide increased security on flights bound for America.

Officials in the US said the airlines targeted were United, American and Continental, which make flights to major airports in New York, Washington and California.

Attorney General Alberto Gonzales said the operation could potentially have killed “hundreds of innocent people“.

He said those arrested overnight were “extremists” who had “gone beyond just stating a desire to kill Americans“.

“Their plotting turned to action as they took several steps to carry out their deadly plan,” Mr Gonzales said.

Mr Chertoff said there was currently no indication of any plotting in the US, but America was taking precautions because it could not assume every alleged member had been rounded up.

The administration has raised the threat level for flights from Britain to “red“, designating a severe risk of terrorist attacks, while all other flights were on “orange“, one step below the highest level.

The US has banned all liquids and gels from flights, including toothpaste, makeup and suntan lotion.

“We believe that the arrests in Britain have significantly disrupted this major threat, but we cannot assume that the threat has been completely thwarted or that we have fully identified and neutralised every member of this terrorist network,” Mr Chertoff said.

He said there was nothing to suggest the target date was September 11.

But he added: “Nor can I tell you that they would have waited that long.”

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

Authorities in the UK had been working on the case for “some considerable period of time” but the fact that flights to the US were being targeted had only become apparent in the past fortnight or less.

“This is not a case where this was just in the initial thought stage – there were very concrete steps under way to execute elements of this plan,” Mr Chertoff said.

The alleged plot was “about as sophisticated as any we have seen in recent years as far as terrorism is concerned“.

He added: “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.”

America’s move was the first time yet it had raised an airline threat status to the highest level.

Passengers arriving into the US from the UK will undergo rigorous inspections because of heightened security checks.

US Customs and Border Protection (CBP) said it had taken a number of “heightened protective measures” to “identify other potential high-risk individuals“.

These included intensive passenger screening of all flights scheduled to travel from the UK to America, before planes leave the gate at British airports.

Manifest information – lists of passengers on the flight – would undergo risk-based screening.

And passengers on flights from Britain and all other international flights would have heightened inspections via searches, X-rays and sniffer dogs when they arrived in the US.