The intelligence service MI6 has restricted all leave until after this summer's Olympic Games in London, according to Sky News sources.
Sky's defence and security editor Sam Kiley said it is not in response to an impending threat from abroad, but is a precautionary measure.
He believes MI5, which is responsible for maintaining internal security, is likely to follow suit.
The Ministry of Defence has said 13,500 security personnel will be involved in the security operation for the Games, which begin on July 29.
They run for just over two weeks until August 12 and are followed by the Paralympics, which run from August 29 to September 9.
The news comes after four radical Islamists admitted an al Qaeda-inspired plot to bomb the London Stock Exchange.
The Muslim fundamentalists' group wanted to send five mail bombs to various targets during the run up to Christmas 2010 and discussed launching a "Mumbai-style" atrocity.
A hand-written target list found at one of the defendant's homes listed the names and addresses of London Mayor Boris Johnson, two rabbis, the American Embassy and the Stock Exchange.
They and five other men who pleaded guilty to a variety of terror crimes at Woolwich Crown Court will be sentenced next week.
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