More than 1,000 troops who had been on standby to bolster security at the Olympic Games have been deployed.
Culture Secretary Jeremy Hunt said the 1,200 troops were to be used because ministers were clear that "we don't want to leave anything to chance".
Earlier this month 3,500 personnel were drafted in after security provider G4S admitted it was short of staff.
The new deployment decision was taken at a meeting of the cabinet's Olympics committee, chaired by David Cameron.
It is the latest step to strengthen security at the Games after G4S said it could not deliver enough guards.
Mr Hunt told an Olympics security briefing the next 48 hours were "critical" for security and transport".
Earlier, he said that the move to bring in extra troops was not because G4S's performance had deteriorated - he said it had actually improved.
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"With three days to go until the opening ceremony, with an incredibly busy weekend, we don't want to leave anything to chance and we just decided that this is the right measure to take because for the public the most important thing is a safe and secure Games," he said.
"It's better to have those troops on the ground so that, were they to be needed, they can swing into action immediately."
Metropolitan Police Assistant Commissioner Chris Allison said the deployment of further troops also offered extra flexibility.
"We don't know what's going to come round the corner, and that's the reality," said Mr Allison, who is the national Olympic security co-ordinator.
Paul Deighton - chief executive of Olympic organisers Locog - said G4S had just under 6,000 personnel deployed on the Games to date.