Britain raised its terrorism threat level on Friday to severe, the second highest level, 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 travelled there from the UK and Europe to take part in those conflicts.
The murder of US journalist James Foley almost two weeks ago by a man suspected of being a British national 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.