The security of the Olympic Games cannot be guaranteed and the unexpected threat could be the most dangerous, the head of MI5 has said.
Jonathan Evans, director general of the Security Service, said there was no doubt that some terrorist groups had considered whether they could pull off an attack on Britain this summer.
He insisted the Games were not an easy target, but added that there was "no such thing as guaranteed security".
"The dog you haven't seen may turn out to be the one that bites you," he said.
Giving his first public speech in almost two years, just one month before the Olympics begin, Mr Evans said the Games were "at the forefront of our minds".
"The preparations have gone well," he said.
"But planning for the future is always planning for uncertainty.
"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.
"But the Games are not an easy target and the fact that we have disrupted multiple terrorist plots here and abroad in recent years demonstrates that the UK as a whole is not an easy target for terrorism."
Lone individuals sympathetic to al Qaida's cause are believed to be one of the major concerns being considered by the security services and police in the run-up to the Games, it is understood.
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.
And no separate Olympic security unit has been set up.
The threat to the UK from international terrorism is currently rated substantial - the third highest of five levels - and will not be increased simply because the Games are getting closer.
If it does rise, it will be because specific intelligence indicates the threat has increased, perhaps because of a plot or because of a number of different circumstances which have raised the risk.
The rating is set by the Joint Terrorism Analysis Centre (Jtac), based at MI5's headquarters at Thames House in central London, but is independent of the service.
In planning for the Olympics, it was presumed the level would be at the second highest, severe, which was last in place last July.
Mr Evans went on: "A lot of hard work still lies ahead and there is no such thing as guaranteed security.
"But I think that we shall see a successful and memorable Games this summer in London."