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."
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".