Authorities lowered the threat level but warned Britons to remain vigilant, saying they may still be the targets of Islamic terrorists after security forces foiled an alleged plot to bring down packed trans-Atlantic planes.
The threat level was reduced from critical to severe, reflecting an intelligence assessment that a terrorist attack remained highly likely but was no longer imminent.
"I want to stress ... that the change in the threat level does not mean that the threat has gone away," UK Home Secretary John Reid said at an early-morning news conference.
"There is still a very serious threat of an attack. The threat level is at severe, indicating the high likelihood of an attempted terrorist attack at some stage, and I urge the public to remain vigilant," he added.
Those sentiments were echoed across the Atlantic by US Homeland Security chief Michael Chertoff, who said there was a risk that other groups might try to cause bloodshed on the false assumption that law enforcement and intelligence services might be distracted.
He also called for taking a renewed look at US laws that could give authorities the flexibility to hold suspects for longer periods of time. Britain recently passed controversial legislation giving the government up to 28 days to hold terror suspects without charge, and the jetliner plot is the first major test of how those new powers will be used.
With the reduction in the threat level, the Department of Transport said that passengers would be allowed to carry a single, briefcase-sized bag aboard aircraft, and that books, laptop computers and iPods would be permitted again.
However, Heathrow and other major London airports said they would not adopt the relaxed regime until today.
British police on Sunday were questioning 22 of the suspects in detention, but authorities remained silent on what, if anything, they have learned. For the third straight day, there were no briefings by police or government officials, leaving the British press to speculate on a wide range of theories.
The Sunday Mirror tabloid asserted that a female suspect in custody may have been planning to use her own baby as a deadly diversion to smuggle a bomb onto the plane, but it did not name its sources. The Sunday Times reported that one of those in custody was believed to be al-Qaeda's leader in Britain, but it did not say which of the suspects it was.
There was plenty of time for travelers to soak up all of those theories as they formed long lines at airports in the UK, particularly London's Heathrow and Gatwick. Almost a third of flights out of Heathrow were canceled on Sunday. British Airways canceled almost 100 flights to Europe from Heathrow and scrapped all its domestic flights from Gatwick. Most long-haul flights were operating, although 10 BA services to the US were canceled.
Some airlines have accused BAA PLC -- which operates Heathrow, Gatwick and Stansted airports serving London -- of failing to cope with the new anti-terror security requirements, and others appealed to the British government to use police and army reservists to speed up searches at overloaded airport security checkpoints.
"If we the industry and the government don't work together to have sensible security ... we are going to hand these extremists a terrific PR success," Michael O'Leary, the chief executive of budget airline Ryanair, told Sky News television. "We don't need to be body searching young children traveling with their parents on holiday to Spain."