REUTERS/Suzanne Plunkett
Britain's Home Secretary Theresa May arrives at Number 10 Downing Street in London August 29, 2013. Prime Minister David Cameron said on Thursday it was "unthinkable" that Britain would launch military action against Syria to punish and deter it from chemical weapons use if there was strong opposition at the United Nations Security Council.
It's the fourth-highest of five potential threat levels. According to the U.K. government, it means an attack is "highly likely."
According to Sky News, May said the threat level was raised in general because of the deteriorating situation in Iraq and Syria, where extremist militants from the group calling itself the Islamic State (also ISIS or ISIL). May said, however, that there was no specific threat to Britain that caused the raise in the terror level.
"The increase in the threat level is related to developments in Syria and Iraq where terrorist groups are planning attacks against the West," May said.
"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."