Passengers arriving into the US from the UK will now undergo rigorous inspections because of heightened security checks, officials said last night.

The move came as the US issued its highest terrorism alert for commercial flights from Britain and raised security for all other air travel.

US Customs and Border Protection 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.

A statement said: "CBP will increase enforcement efforts in international arrival areas, using risk-based targeting and deploying special response teams including baggage and aircraft search teams, baggage X-ray equipment, specially trained canine units, and explosive detection technology."

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

Homeland Security Secretary Michael Chertoff said the would-be terrorists had planned to carry the explosive material and detonating devices on to planes disguised as drinks and electronic devices.

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.

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

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 w as reminiscent of one hatched by September 11 mastermind Khalid Sheikh Mohammed in the 1990s to detonate bombs on airliners travelling over the Pacific Ocean.