LONDON (AP) - Britain raised its terror threat alert to the second-highest level Friday, one of several recent moves the country has made to increase vigilance against international terrorists after a Christmas Day bombing attempt on a Europe-U.S. flight.
The threat level was raised from "substantial" - where it had stood since July to indicate a strong possibility of a terrorist attack - to "severe," meaning such an attack is considered highly likely.
In making the announcement, Home Secretary Alan Johnson said the raised security level means that Britain is heightening its vigilance. But he stressed that there was no intelligence suggesting an attack is imminent.
"The highest security alert is 'critical,' and that means an attack is imminent, and we are not at that level," he said on British television.
Johnson declined to say what intelligence the change was based on, or whether the move was related to the failed Christmas bombing attempt, when U.S. authorities say a young Nigerian named Umar Farouk Abdulmutallab tried to detonate a bomb hidden in his underwear during a flight from Amsterdam to Detroit. Abdulmutallab, who allegedly had links to extremists based in Yemen, had studied as a university student in London.
"It shouldn't be thought to be linked to Detroit, or anywhere else for that matter," Johnson said. "We never say what the intelligence is."
He said the decision to raise the threat level was made by the U.K.'s Joint Terrorism Analysis Center. He said the center kept the security threat level under constant review and made its judgments based on a range of factors, including the "intent and capabilities of international terrorist groups in the U.K. and overseas."
Earlier this week Britain suspended direct flights to Yemen's capital in response to the growing threat from al-Qaida-affiliated militants based in that country. Prime Minister Gordon Brown said his government also was creating a new terrorist no-fly list and targeting specific airline passengers for tougher security checks.
The measures followed a discussion between Brown and President Barack Obama on Tuesday.
Brown said Britain and other nations face a sharply growing threat from al-Qaida-affiliated terrorists based in Yemen and an area of north Africa that includes nations such as Somalia, Nigeria, Sudan and Ethiopia.
Britain's five-tiered alert system - which starts at "low" and passes through "moderate," "substantial," and "severe" before hitting "critical" - is similar to the U.S. system of color-coded terrorism advisories.
The British government downgraded the alert level to "substantial" in July without explaining the decision. The level briefly rose to "critical" in 2007, after authorities foiled an alleged plot to blow up a North America-bound aircraft in August 2006 and botched car bomb attacks in London and Scotland.
In the United States, the alert level for the aviation sector is currently at "orange," indicating a high risk of terrorist attacks. It has not been changed since 2006, after terrorist plans to blow up jetliners en route to the U.S. from Britain were discovered. The alert level for the rest of the country is at "yellow," indicating a significant risk.
Last week the U.S. increased security at airports and on planes as intelligence officials warned that al-Qaida's branch in Yemen was continuing to plot attacks on the United States.
At the time, a counterterrorism official speaking on condition of anonymity said the move was spurred by intercepted communications surrounding terrorists in Yemen, but the official cautioned that there is a steady stream of threat information emanating from there.
The stepped-up security included more air marshals on flights to and within the U.S. and more random additional screening at airports around the world.
Britain's decision to raise its terror threat alert came as India put airline passengers through extra security screenings and sky marshals were placed on flights. India put its airports on high alert amid reports that al-Qaida-linked militants planned to hijack a plane.
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Associated Press Writers Devlin Barrett and Eileen Sullivan contributed to this report from Washington.