The British government downgraded its terror threat level from critical to severe Monday, saying intelligence suggested an attack was no longer imminent after security forces foiled an alleged plot to bring down packed trans-Atlantic planes heading to the United States.

The critical level was declared last week when police rounded up 23 people suspected of involvement in the terror plot. Police questioned all but one of the suspects Sunday, but authorities remained silent on what, if anything, they have learned.

"I want to stress ... that the change in the threat level does not mean that the threat has gone away," Home Secretary John Reid said Monday.

"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.

CountryWatch: Great Britain

Those sentiments were echoed across the Atlantic by U.S. Homeland Security chief Michael Chertoff, who said Sunday 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 U.S. laws that could give authorities the flexibility to detain 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 utilized.

With the reduction in the threat level, the Department of Transport said 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 airports said they would not adopt the relaxed regulations until Tuesday.

Reid said the downgraded terror threat level was not a response to airport congestion and flight cancellations.

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."

Almost a third of flights out of Heathrow were canceled Sunday — the airport handles about 1,250 flights a day. 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 flights to the United States were canceled.

Police arrested 24 people across England on Thursday, saying they had thwarted a plot to blow up as many as 10 passenger planes flying between Britain and the United States. One suspect was released without charge, and a court will decide Monday on the detention of another. That last suspect cannot be questioned in the interim.

Another 17 people are in detention in Pakistan — including Rashid Rauf, a British national named by Pakistani intelligence as one of the key suspects. Rauf was picked up along the Pakistan-Afghanistan border, and is believed to have connections to a senior al-Qaida leader in Afghanistan.

In Kabul, Afghanistan's Foreign Ministry on Sunday denied any Afghan connection, saying the country — home to thousands of NATO and American troops — was no longer a safe place for Al Qaeda to operate.