Britain raised its international terrorism threat level to the second highest level of "severe" on Friday in response to possible attacks being planned in Syria and Iraq, Home Secretary (interior minister) Theresa May said.
"That means that a terrorist attack is highly likely, but there is no intelligence to suggest that an attack is imminent," May said in a statement.
"The increase in the threat level is related to developments in Syria and Iraq where terrorist groups are planning attacks against the West. Some of those plots are likely to involve foreign fighters who have traveled there from the UK and Europe to take part in those conflicts."
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It is the first time since mid-2011 that Britain has been placed on this grade of alert following an assessment carried out by the Joint Terrorism Analysis Center, the body responsible for setting the national threat level.
It comes less than two weeks after a video released by Islamic State (IS) showed the beheading of U.S. journalist James Foley, apparently by a masked knifeman speaking English with a London accent. An investigation to identify the suspected attacker is ongoing.
Foley's gruesome murder prompted demands for extra security measures to tackle Britons traveling to the Middle East to join militant groups after officials again warned that some of those who had gone to Syria or Iraq to fight might return to Britain to carry out attacks.
Soon afterwards, May promised tougher new laws against Islamists to stop them going abroad and to tackle radicalisation among Britain's 2.7 million Muslims.
"We have already taken steps to improve our powers and increase our capabilities for dealing with the developing terrorist threats we face," May said on Friday.
"That process will continue and the British public should be in no doubt that we will take the strongest possible action to protect our national security."
The national threat level was first published in August 2006, just over a year after four British Islamists carried out suicide bombings on London's transport network killing 52 people.
It has twice been raised to the highest level of critical - meaning an attack is imminent - after a plot to blow up trans-atlantic airliners was thwarted in 2006 and the following year following attempted car bombings in London and Glasgow.
Security chiefs say they have managed to stop at least one major terrorism plot every year since the 2005 bombings, known as the 7/7 attacks, but last year an off-duty soldier was murdered on a London street by two British Muslim converts in what the government described as a terrorist killing.
Officials estimate at least 500 Britons have traveled to Syria or Iraq, where IS has seized large swathes of territory, and London's police chief Bernard Hogan-Howe said on Wednesday it was believed some 250 had since returned.
A day earlier, Britain's top counter-terrorism police officer Mark Rowley said there had been 69 arrests linked to fighting in Syria this year, a fivefold rise in the arrest rate compared to last year.
"The growth of dangerous individuals poses challenges for policing, especially when nearly half of Syria travelers of concern were not known as terrorist risks previously," he said.