MI5 director general Jonathan Evans also warned that some terrorist groups would be considering whether they could launch an attack on Britain during this summer's Olympic games.
The Games present an attractive target for our enemies and they will be at the centre of the world's attention in a month or so.
No doubt some terrorist networks have thought about whether they could pull off an attack.
He insisted the Games were not an easy target, but added that there was "no such thing as guaranteed security".
All leave has been suspended at MI5 to ensure the service has its full complement of some 3,800 staff for the Olympics, double the number it had a decade ago after the September 11 terror attacks.