* Organisers to employ nearly 24,000 guards at venues
* Total cost of security now more than $1.5 billion
* Spending on opening and closing ceremonies also doubles
By Tim Castle
LONDON, Dec 5 (Reuters) - Britain has doubled its budget for security at venues for next year's London Olympics after underestimating the number of guards needed to admit spectators, athletes and officials through airline-style security, organisers said on Monday.
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 inside of the Olympic and Paralympic venues.
The four ceremonies for the opening and closing of the Olympics and Paralympics will also have their funding doubles with 41 million pounds added to the 40 million pounds already planned.
Sports Minister Hugh Robertson said the London organising committee's (LOCOG's) original estimate of guard numbers had been made before the final design of the venues became clear and the publication of the competition schedules.
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 mean the venue security budget will jump to 553 million pounds from the previous estimate of 282 million, organisers said in the latest 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 possible attraction of the Games to militants determined to publicise their cause means the British government can take no chances with security.
The Olympic Park in Stratford, east London, may even be protected by missiles against airborne attack, in line with measures taken in Beijing in 2008 and Athens in 2004, Defence Minister Philip Hammond said last month.
The Games coincide with the 40th anniversary of the Munich Olympics, when 11 Israeli team members died after being held hostage by Palestinian gunmen.
Britain has been a target for many years, with its role in Iraq and Afghanistan as a leading U.S. ally increasing the threat from Islamic militants.
Robertson acknowledged the sensitivity of giving extra state funds for Olympic ceremonies when Britain was reducing spending elsewhere in a tight austerity programme, but said it was justified by their potential economic impact.
Four billion people around the world are expected to watch the ceremonies, delivering publicity that would cost a "stratospheric" amount of money to buy, Robertson said.
"It seemed foolish given that backdrop not to provide the extra investment...in order to maximise that one-off global opportunity," he added .
Oscar-winning film maker Danny Boyle is artistic director for the opening ceremony.