Britain's government reduced its terror alert level Monday to its lowest level since the July 7, 2005 bombings of the London transport system but warned that the threat remains serious.

The government did not explain its decision to downgrade the threat from "severe," which Britain's domestic spy agency MI5 describes on its Web site as a "high likelihood" of future terror attacks" to "substantial," which means that such an attack remains a "strong possibility."

Terror experts were divided on the thinking that lay behind the change.

Bob Ayers, a London-based former U.S. intelligence officer, said officials may have reduced the threat level in an effort to show that the war in Afghanistan — where British soldiers are engaged in an increasingly bloody fight with Taliban rebels — is helping keep the country safe from extremists. But he said the move sat uneasily with dire warnings voiced last year that British security services were tracking 2,000 people and dozens of suspected terrorist plots.

"Where did they go? Have they just disappeared?" Ayers asked.

Paul Wilkinson, a terrorism expert at the University of St. Andrew's in Scotland, rejected suggestions the move was politically inspired, saying Britons should treat the downgrade as a "very cautious, very tentative reduction" which could be reversed at any time.

Britain's Home Office announced the new alert level in a brief e-mail to journalists. It did not detail the reason for the change, saying only that its analysis center makes judgments based on a broad range of factors.

"We still face a real and serious threat from terrorists and the public will notice little difference in the security measures that are in place, and I urge the public to remain vigilant," Home Secretary Alan Johnson said in the e-mail.

Britain's five-tiered alert system — which starts at "Low" and passes through "Moderate," "Substantial," and "Severe" before hitting "Critical" — is similar to the United States' system of color-coded terrorism advisories, which has been criticized as vague.

Last week the Obama administration said it would review the system, a move that could spell an end to the advisories.

Britain's Home Office said there were no plans to scrap or review its system.

Britain's previous, secret alert system, which worked on a slightly different scale, was overhauled and made public after a report showed it had little effect on police preparedness before the London transit bombings, in which four suicide bombers killed 52 people.

That investigation also revealed that the alert level had been dropped to "Substantial" only two months before the attacks.

The new British alert system made its debut at "Severe" and it has largely remained there since, although it briefly rose to "Critical" after authorities foiled an alleged plot to blow up North America-bound aircraft in August 2006 and botched car bomb attacks in London and Scotland in June 2007.

There had been hints over the past months that the British threat level would fall. In March, Britain's then-Home Secretary Jacqui Smith said support for al-Qaida among Muslims worldwide was declining. Speaking before senior police officers in Manchester earlier this month, the country's top counterterrorism officer John Yates said Britain's threat level would possibly be reduced.

"It is logical because we cannot keep having it high unless the threat is there," he said.