Britain and the United States lowered alert levels a notch Monday and lifted some restrictions for air passengers after British officials said they were holding the main suspects in an alleged Al-Qaida plot.

But the authorities in London warned that the threat of an attack had not disappeared and admitted that there would be confusion at airports as officials grappled with the new security measures.

The British Government early Monday downgraded the alert level to "severe," the fourth highest of five levels, from an unprecedented "critical," where it had stood since police announced on Thursday it had foiled a major plot.

Following the British decision, the US Department of Homeland Security announced it was downgrading the threat level for inbound flights from Britain to "code orange" from "code red," its highest security risk alert.

On Thursday, British authorities arrested two dozen men suspected of plotting to smuggle volatile chemicals on to several passenger jets headed for the United States with the intent to set off explosions in mid-flight.

Britain's Home Secretary John Reid said that the threat of attack was no longer imminent as police "believe that the main suspects in the alleged plot were arrested last week."

But he warned that an attack remained "highly likely" as police investigated other suspected plots.

Transport Secretary Douglas Alexander said the new security picture meant that passengers would now be able to take aboard one item of hand luggage, though a ban on non-essential liquids and gels would remain in place.

BAA, the operator at Heathrow Airport, said it was setting up "the appropriate procedures" to implement the new security requirements but the hand baggage ban would remain at Heathrow until today.

US Homeland Security Secretary Michael Chertoff issued warnings similar to Reid's.

"Let me be clear: this does not mean the threat is over," he said.

He also told Fox News on Sunday that US and British investigators were still digging to see whether or not the plot broken up on Thursday was directed by Al-Qaida.

At London's Heathrow airport, Europe's busiest air hub, there was no sign of an immediate easing of restrictions.

BAA said passengers traveling from Heathrow should still check with their airline before leaving for the airport, arrive with no hand luggage, bring only those items allowed into the cabin in a clear plastic bag and be prepared for delays.

News of the alleged plot caused air travel chaos around the world as the cancellations of flights caused disruptions elsewhere and other countries also tightened security measures.

Despite the increased security checks passengers were still apparently able to get some banned items on board, with a mobile phone causing a scare on a British Airways flight from London to New York on Sunday.

The plane turned around mid-flight and returned to Heathrow for a full security search after the phone started ringing and no passenger claimed it, the airline said.

(China Daily August 15, 2006)