Money being spent on security has now blown out to £553 million, with 23,700 security staff instead of the 10,000 originally planned, the government has confirmed.
The opening ceremonies are costing taxpayers an extra £41m alone.
Olympics minister Hugh Robertson confirmed the uncertain global situation was the reason.
He said: “When I started being minister there was no Arab Spring. No one really knows whether that’s going to have a beneficial or adverse effect.
“This is what is needed to provide a safe and secure Games.”
The security plans will cover 100 venues with 30 control rooms. There will be around 2000 X-ray machines and metal detectors, plus 40 miles of security fencing.
The security staff will come from private firms, the military, volunteers and a training scheme.