Britain's GCHQ eavesdropping agency has for the first time deployed armed police guards in front of its headquarters outside London, a spokeswoman said, citing the heightened risk of an attack by militants.
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."
With more than 6000 employees, GCHQ - the equivalent of America's National Security Agency (NSA) - is based in a futuristic building in Cheltenham, about 160km from London, named the doughnut because of its shape.
The decision to beef up its defences, designed to be a visible deterrent, follows a similar move to deploy armed soldiers at a top tourist attraction in the heart of London's government district last October as a precautionary measure.
