Surface-to-air missiles could be used to protect the skies over London during the Olympics, Defence Secretary Philip Hammond has said.
He insisted that "all necessary measures" will be taken to ensure security, telling the Commons "appropriate ground-to-air defences" could be in place if that was recommended by the military.
The statement came as Metropolitan Police Assistant Commissioner Chris Allison, the officer in charge of policing at the London 2012 Games, said he "did not recognise" suggestions the US is to send 1,000 agents, including 500 from the FBI, because security will be inadequate.
Former defence secretary Liam Fox, who raised the issue of Olympics security with his successor, noted that surface-to-air missiles had been used at Olympic Games since Atlanta in 1996.
He asked Mr Hammond to confirm "there will be a full level of multi-layered defence and deterrence for the London Games, including ground-to-air based missiles in London".
Mr Allison, the national Olympic security co-ordinator, also denied that top-level security talks are suffering from friction and diplomatic meddling from the US.
"I feel great support from the Americans," he said. "The Games will be delivered by the British police service working with the event organisers Locog (the London 2012 organisers). We will have some support from other colleagues up and down the country but it is the British police who will be doing it."
The US Embassy insisted it had the "utmost confidence" in the security arrangements for the London Games.
In a letter to the Guardian, the US Embassy's Charge d'Affaires Barbara Stephenson said it was "entirely normal and prudent for the US to engage in discussions with UK officials about security arrangements" and there was an "excellent" relationship with the Met.
She said: "The United States Embassy has the utmost confidence in the British Government's arrangements to ensure safety and security for the Olympic Games. The US team is one of the largest participating in the Games, and thousands of American tourists are expected to attend Olympic events. It is, therefore, entirely normal and prudent for the US to engage in discussions with UK officials about security arrangements for the Olympic Games."