Surveillance security

Britain raised its terrorism alert last August to the second-highest level with Prime Minister David Cameron saying the Islamic State group operating in Syria and Iraq posed the country's greatest security risk.

"The security posture of our sites, including the safety of our staff, is under constant review," a GCHQ spokeswoman said.

"Our enhanced security arrangements are not in response to any specific threat but related to the raising of the UK threat level from international terrorism from substantial to severe last year."

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