Police officers patrol outside the U.S. embassy in London December 9, 2014.

Britain faces the threat of a Sydney-style ‘lone wolf’ attack at any moment, Prime Minister David Cameron said on Tuesday, after a 16-hour siege at a cafe in the Australian city in which two captives were killed.

Britain raised it terrorism alert to the second-highest level in August and last month said it was facing the biggest terrorism threat in its history because of radicalized Britons returning from fighting in Syria and Iraq.

“The threat we face definitely includes those sort of self-starting, sometimes quite random attacks that could happen at any moment in Britain,” Cameron told a committee of lawmakers when asked how close Britain was to a Sydney-style attack.

“People who are self-radicalized on the Internet who then suddenly do appalling things, that is much more difficult to prevent.”

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