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.
It comes less than two weeks after a video released by Islamic State (IS) showed the beheading of US journalist James Foley by a masked knifeman speaking English with a London accent.
Foleys gruesome murder prompted demands for extra security measures to tackle Britons travelling to the Middle East to join militant groups.