Ground-to-air missiles may be deployed to protect London's airspace during the Olympic games, Defence Secretary Philip Hammond has said.

Former defence secretary Liam Fox asked his successor to confirm the plans at Hammond's first session of Defence Questions.

The missiles will only be deployed if deemed operationally necessary by the military, Hammond said.

The admission comes after reports emerged that the United States is preparing to send 1,000 security agents, including 500 from the FBI, to protect its athletes and diplomats at the games.

US officials are concerned that the police response to the summer riots was inadequate, and that there will not be enough security staff at the event.

"If something happens and we say we did not have enough people we are finished," the Guardian's source reportedly said.

Organisers originally estimated that around 10,000 security guards would be needed, but it has now determined more than twice that number will be required.

A Home Office spokesman said: "Security planning is on track and funding has been protected. The Government is committed to delivering a safe and secure Games that London, the UK and the world can enjoy.

"The International Olympic Committee (IOC) undertake detailed inspections of security preparations and have full confidence in our plans. The UK has a strong and close working relationship with the US, who have expressed similar confidence.

"The Government, London 2012 Organising Committee (LOCOG) and G4S are working together to finalise the requirement for venue security and as is common at major events in the UK we will make the best and most appropriate use of all available resources."