Britain raised its terrorism threat level to 'severe' -- the second highest level -- on Friday, days before London hosts major international meetings on how to deal with militancy in Afghanistan and Yemen, Reuters reported.
The decision to raise the level from 'substantial' means security services now consider an attack in Britain, a key U.S. ally, to be "highly likely" but the government said it had no information to suggest an attack was imminent.
Britain gave no reason for the move by its Joint Terrorism Analysis Center (JTAC) which comes as Britain and other countries step up precautions in the wake of the failed Christmas Day attack on an airliner in Detroit.
Raising the threat level is expected to lead to tighter security at airports and public buildings.
"JTAC keeps the threat level under constant review and makes its judgments based on a broad range of factors, including the intent and capabilities of international terrorist groups in the UK and overseas," Home Secretary (interior minister) Alan Johnson said in a statement.
"The fact that we've moved to another threat level means we put more resources in, we heighten the state of vigilance. It shouldn't be thought to be linked to Detroit or anywhere else for that matter," he told the BBC.
Security expert Anthony Glees said his guess was that the decision to raise the threat level was linked to the Afghanistan conference and to intelligence from the United States.
"I think it's very probable that people, either members of al Qaeda or associated to al Qaeda, will be figuring that it would be a huge trophy attack in some way to damage the holding of the Afghanistan conference," he told the BBC.
The threat level was last changed on July 20, 2009 when it was lowered to substantial from severe. It had been lowered to severe on July 4, 2007 from critical, the highest level, which had been declared a few days earlier following attempted car bombings in London and Glasgow.
Suicide bombings in July 2005 killed 52 people on London's transport networks and a number of plots have been thwarted since then.