The Ministry of Defence says that similar missile defence systems have been deployed at all recent Olympic Games.
They will be in place by mid-July, along with Royal Navy helicopter carrier HMS Ocean (which will be moored in the River Thames), Royal Air Force Typhoon jets (which will be temporarily stationed at RAF Northolt), and Puma helicopters (at a Territorial Army centre in Ilford).
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The Hammers moved to the Olympic Stadium in May leaving their old site to be developed for housing.
The first officer shouted "look!" as the drone passed next to the right wing at around 11,000ft above south-east London.
They found a sheepish looking ewe with a traffic cone on her head in the middle of a field.