The British government has raised its terror alert level from "substantial" to "severe," the second highest level, but it's not divulging the exact reason behind the decision.

Home Secretary Alan Johnson made the announcement Friday. He said it means his government considers an attack on British soil is likely and that security officials are heightening their vigilance. However, he stressed there is no intelligence suggesting an attack is imminent.

"The highest level of security alert is 'critical,' and that means an attack is imminent, and we're not at that level," Johnson said on British television.

"Yes, we have to be more alert, and it means that an attack has now moved to the level of being likely. We have absolutely no intelligence to suggest that it's imminent," he added.

The decision to raise the threat level was made by the country's Joint Terrorism Analysis Centre, consisting of intelligence officials and police chiefs.

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 Nigerian named Umar Farouk Abdulmutallab tried to detonate a bomb 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."

The announcement came days after Britain suspended direct flights to Yemen's capital in response to the growing threat from al-Qaeda-affiliated militants based in that country.

With files from The Associated Press