The games are Britain’s largest peacetime security operation and keeping the competition safe now looks set to cost more than a billion pounds out of the event’s total 9.3 billion pound ($14.5 billion) budget.

An extra 13,700 guards, including an unspecified number of soldiers and volunteers, will be employed on top of the 10,000 already planned to secure the perimeters of the Olympic and Paralympic venues.

The original estimate of guard numbers had been made before the final design of the venues became clear and the publication of the competition schedules, Sports Minister Hugh Robertson said.

Increased uncertainty over international security after the Arab Spring uprisings and other events had also been a factor in the jump in the guard force.

“When I started being a minister 18 months ago, there was no Arab Spring, for example. Nobody necessarily knows whether that is going to have a beneficial or an adverse effect,” Robertson told reporters.

“The situation is pretty fluid and that is what sometimes makes some of this planning difficult.”

The extra staff means the venue security budget will jump to 553 million pounds from the previous estimate of 282 million, organizers said in a quarterly financial report.

“That is the amount that we think is absolutely necessary to deliver a safe and secure Games,” Robertson added.

The spending increase comes on top of the 475 million pounds already planned for policing and other security measures outside stadiums, with 12,000 officers on duty at peak during the games.

The attraction of the Games to militants determined to publicize their cause means the British government can take no chances with security.

The main Olympic venue in London may even be protected by missiles against airborne attacks, in line with measures taken in Beijing in 2008 and Athens in 2004, Defense Minister Philip Hammond said last month.

Britain has been a terrorist target for many years, with its role in Iraq and Afghanistan as a leading US ally increasing the threat from militants.