Britain hiked its terrorism threat level on Friday to "severe", the second-highest level 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 U.K. and Europe to take part in those conflicts."
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In a hastily convened press conference, Prime Minister David Cameron said the risk from terrorism in the U.K. was at a "three-year high".
In particular, he said the risk from ISIS (also known as the Islamic State) was higher "than we have ever known before" and that there was "no doubt in his mind" that the Islamic extremist group was targeting Western Europe.
Cameron pledged that he would do whatever needed to keep the British people safe. New legislation, due to be released in more detail on Monday, should make it harder for Britons to travel to Syria and Iraq to fight alongside militants.
"We know terrorist organizations thrive where there is political instability...It has become clear that there are some gaps in our armory and we need to stop them," he said in his press conference outside his official residence in Downing Street.
The murder of U.S. journalist James Foley almost two weeks ago by a man suspected of being a British member of ISIS has prompted demands for extra security measures to tackle Britons travelling to the Middle East to join militant groups.
It also raised fears that some of those who had gone to Syria or Iraq to fight would return to Britain and carry out attacks.
The White House said it had no plans to change the U.S. terror level in response to the U.K. alert.