The Unites States of America are to send 1,000 Agents, 500 from the FBI to the London 2012 Olympics amid fears of inadequate British.

The news came as Defence Secretary Philip Hammond said surface-to-air missiles could be used to protect the capital’s skies.

Mr Hammond, 55, told the Commons “appropriate ground-to-air defences” could be in put in place if recommended by the military. 

US intelligence chiefs suspect al-Qaida is plotting an attack on the Games, with American athletes their top target.

The decision to bring over hundreds of FBI agents comes after the US believe British organisers have massively underestimated the threat posed by terrorists.

The London Organising Committee of the Olympic and Paralympic Games originally thought that 10,000 guards would be enough, but after a review this summer showed they will need up to 21,000 security officers to protect the 32 sites across the UK.

Security firm G4S has won the contract to police the venues but is running out of time to fill the huge shortfall.