RAF E-3D airborne warning and control system (AWACS) aircraft are overseeing British airspace during the London Olympics.
NATO E-3A Component photo
It is not the first AWACS mission to provide surveillance and early warning against terrorist threats during a sporting event or the Olympics. NATO E-3As guarded Greek airspace during the 2004 Athens Olympics and most recently the European soccer championship games played in Poland (Ukraine is not a NATO member).
Although the RAF E-3Ds are part of the NATO AWACS force, they are overseeing the Olympics under their national rather than the NATO flag.