The Defence Secretary, Philip Hammond, said yesterday that surface-to-air missiles could be in place to protect the 2012 Olympics, as the Government insisted its plans could cope with the country's biggest security operation for decades.
Mr Hammond's statement in the House of Commons appeared intended to ease concerns after US disquiet with the arrangements. It is holding intensive talks with Britain and plans to send some of its own agents.
Initial estimates of the number of private security guards needed for the 34 venues have also proved to be far too low and soldiers are likely to be used to fill the shortfall.
The US and Israel have raised concerns over security at previous Olympics, notably in Greece, when pre-Games tests revealed major problems. An agent disguised as a pregnant woman smuggled a fake bomb during one test of security procedures.
The Israeli Foreign Ministry yesterday declined to say whether it was working with the British authorities on security for its own athletes. "That's not something we want to discuss," a spokesman said. "Any exchanges on these issues are limited to a very, very small circle of professionals and that's where it's going to stay."
Police confirmed that a small number of foreign "security liaison officers" would be in Britain for the Games. Security company G4S was given the contract this year to provide some 10,000 security staff and yesterday began advertising for positions to X-ray bags, check CCTV, search people and patrol arenas during 12-hour shifts.
"Experience is beneficial but as long as you can remain calm under pressure, think quickly on your feet, and are good with people, we'd like to hear from you," according to one advert. A spokesman said the company had flexibility to recruit more if required.
The security industry initially estimated that about 10,000 staff would be needed at venues but revised plans in the summer have increased that number to up to 21,000, according to some industry estimates. The Home Office, the Games organising committee, Locog, and police declined to comment on the figure.
The issue of security was raised in the Commons yesterday by the former defence secretary Liam Fox. He said 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".
The Defence Secretary told him: "All necessary measures to ensure the security and safety of the London Olympic Games will be taken, including, if the advice of the military is that it is required, appropriate ground-to-air defences."
The current Home Office risk assessment of attack from international terrorists stands at "substantial" – the third-highest level. The national Olympic security co-ordinator, Chris Allison, of the Metropolitan Police, said that he believed 12,000 police will be needed nationally for the event.