Citing the risk of foreign fighters in Iraq and Syria returning to conduct attacks, 
the UK government raised its terror threat level Friday from substantial to severe.
“That means that a terrorist attack is highly likely, but there is no intelligence to suggest that an attack is imminent…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.” Home Secretary Theresa May said.
Update: U.K. notes they believe at least 500 persons with UK passports have left to fight and could return.
