Surface-to-air missiles could be used to protect athletes and visitors during the London Olympics next year, according to the Defence Secretary Philip Hammond.
The admission came in reply to a question from his predecessor Liam Fox.
"(This will include), if the advice of the military is that it is required, appropriate ground-to-air defences."
Mr Hammond was speaking after it emerged the US may be preparing to use its own agents to protect athletes and diplomats at the Games,. although the Government insists security plans are "on track".
America could send up to 1,000 agents - including 500 from the FBI - to London next summer, according to The Guardian.
The newspaper reported US officials have raised "repeated concerns" about security and are worried the UK has had to restrict the scope of anti-terrorism "stop and search" powers.
They are also reported to be anxious about the police response to the London riots.
Earlier this year, national Olympic security co-ordinator Chris Allison, of the Metropolitan Police, said he believed 12,000 officers will be needed nationally to police the event.
There are also plans to use large numbers of military personnel to police the games - including SAS (Xetra: A1C0DX - news) troops stationed in boats along the Thames in case of a Mumbai-style terror attack.
FBI spokesman Bill Carter did not say how many agents would be dispatched to London 2012 but said "a significant number of agents" were sent to the games in Athens and Beijing.
"It is not unusual for a contingent of agents to be dispatched to the Olympic Games to provide co-ordination and liaison regarding security matters," he said.
Mr Carter added that any US involvement was done with "close co-operation" with the host country.
One former senior FBI agent told Sky News it was "plausible" that FBI agents would be sent to London for the games, although numbers of up to 1,000 agents "seem high".
He said any agents sent to London would probably not be armed.
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 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 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."
Sky News' security editor Sam Kiley said: "Britain is a significant terrorist target... from the American perspective, you can't be too careful."
A Home Office source said it did "not recognise as true" suggestions that there are concerns from the US.
The US state department declined to comment, and the FBI was not available for comment.