Britain will provide up to 13,500 troops to protect next summer's London Olympics -- more than it has in Afghanistan -- after organisers said international uncertainty meant security for the event needed to be doubled.
Defence Secretary Philip Hammond said the military support would provide back-up for police and private staff already hired to secure the perimeters of the Olympic and Paralympic venues in what will be Britain's largest peacetime security operation.
The military presence will also include special forces and specialist bomb disposal units as well as a 1,000-strong contingency force in the event of an Olympics-related civil emergency.
The Olympic Park in Stratford, east London, will be protected by missiles against airborne attack, in line with measures taken by Beijing in 2008 and Athens in 2004.
Next year's Olympic and Paralympic Games are once-in-a-generation events for the UK, Hammond said in a statement.
We want them to be secure, so that all those competing and attending can enjoy the games for the celebration of sporting achievement and cultural celebration that it is.
Increased concern about international security after the Arab Spring uprisings and other events had led to an increase in the budget for venue security to 553 million pounds ($850 million) from the previous estimate of 282 million, Sports Minister Hugh Robertson said earlier this month.
Another 475 million pounds is being spent on policing and other security measures outside stadiums, with 12,000 officers on duty at peak times.
Britain has been a target for Islamist militants for many years as a leading ally of U.S. military action in Iraq and Afghanistan, and 52 people were killed in suicide bombings in the capital in 2005.
The government said in July that the terrorism threat level had been downgraded by one notch to substantial, the third highest category, meaning an attack is a strong possibility.
The military presence for the games will now be bigger than Britain's current deployment in Afghanistan, where there are about 9,500 British troops.
Of the 13,500 military personnel, about 7,500 will help at venues, helping to admit spectators, athletes and officials through airline-style security.
They will combine with guards from private security firm G4S and volunteers to produce a force of 23,700, up from an original estimate of 10,000.
HMS Ocean, the largest ship in the Royal Navy, will be moored in the River Thames at Greenwich, providing a base for military helicopters, while the navy's flagship HMS Bulwark will be deployed off the south coast.
Typhoon fighter jets will also be based at RAF Northolt in west London.
In November, London's national Olympic security coordinator Chris Allison dismissed as rubbish reports that the United States had been unhappy with arrangements for the games and was planning to send up to 1,000 agents to protect its athletes.
The games will coincide with the 40th anniversary of the Munich Olympics, where 11 Israeli team members died after being taken hostage by Palestinian gunmen.
Every Olympics since Atlanta (1996) has had a significant military component around its security, Hammond told BBC TV.
Our allies and our friends would expect us to deliver the same in London and I think both the London public and Olympic competitors and visitors should be greatly reassured by the role that the military are playing.
(Additional reporting by Tim Castle; Editing by Steve Addison)