The new deployment decision was taken at a meeting of the cabinet's Olympics committee, chaired by Prime Minister David Cameron.
Olympics Minister Jeremy Hunt announced that the 1,200 troops were to be used because ministers were clear that "we don't want to leave anything to chance".
The extra troops deployed today, were already standing by on 48 hours' notice.
The total military deployment at the 2012 Olympics is now 18,200. Earlier this month 3,500 personnel were drafted in after security provider G4S admitted it was short of staff.
It is the latest step to strengthen security at the Games after G4S said it could not deliver enough guards, the BBC reported.
Hunt, also a Culture Secretary, said the latest move 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," he added.
Paul Deighton, chief executive of Olympic organisers Locog, said G4S had just under 6,000 personnel deployed on the Games to date.
Some 10,500 athletes will take part in the Games.
Around 11,000 personnel will be performing security duties at the Olympic venues across London and beyond during peak times.