Britain is ready to use missiles to protect next year's London Olympics from an airborne attack, Defence Minister Philip Hammond said on Monday, amid reports the United States was unhappy with security plans for the games.

"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," Hammond told parliament.

It would be the first time surface-to-air missiles have been deployed in Britain since the end of the World War II, a defence ministry spokesman said, adding that no decisions had been made yet.

Hammond's announcement came as the Guardian newspaper reported the United States was concerned about security at the games and planned to send up to 1,000 of its agents, including 500 from the FBI, to protect American contestants and diplomats.

London's Olympic security coordinator Chris Allison dismissed the report, saying he was about to fly to Washington in a prearranged trip to give an update on planning for next summer's event.

"We're not hearing any concerns being expressed from them at all," he told BBC radio, adding that there would only be a small number of foreign security liaison officers at the games.

One option for providing surface-to-air defence in London could include stationing one of Britain's new missile-equipped Royal Navy Type 45 destroyers in the River Thames estuary near the games, the defence ministry spokesman said.

Another choice would be to use the British Army's land-based Rapier surface-to-air missile launcher.

More than 6,000 British soldiers dressed as civilian security guards are expected to boost security at London's Olympic venues, a source told Reuters earlier this month, supplementing force of at least 10,000 civilian guards and 12,000 police.

Britain remains on high alert for a terrorist attack despite its international threat level being lowered from severe to substantial, the third-highest level, meaning an attack is a strong possibility and could occur without warning.

Four young British Islamists killed 52 commuters in suicide bomb attacks on London's transport network in July 2005, the day after the capital was awarded the 2012 games.