Up to 500,000 people from across the world are being screened to try to stop next month's Olympic Games from being targeted by criminals or terrorists.
The stringent security checks were designed to ensure nothing will be left to chance, said the Home Office.
Officials from the Home Office and the Security Service have completed screening about two thirds of the domestic and foreign accreditation applications from athletes, coaches, security staff and media from more than 200 competing nations, 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, it is understood.
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.
A Home Office spokesman said: "We are undertaking stringent checks on all those seeking accreditation. This rigorous process has been designed to ensure those working at the Games are fit to do so."