LONDON, UNITED KINGDOM - AUGUST 29: British Prime Minister David Cameron speaks to the media during a press conference at Downing Street on August 29, 2014 in London, United Kingdom. The Prime Minister is to introduce new legislation to make it harder for Britons to travel to Syria and Iraq to fight for Islamist State (IS). Earlier today, the British Home Secretary Theresa May announced that Britain has raised its international terrorism threat level to 'severe', indicating that a terrorist attack is 'highly likely'.  (Photo by Paul Hackett  - WPA Pool/Getty Images)