An additional 1,200 troops are to be deployed at the Olympics amid continued concern about security staffing.
Ministers took the decision on Tuesday morning at a Cabinet Committee for the Olympics chaired by Prime Minister David Cameron.
Culture Secretary Jeremy Hunt said the numbers to be provided by beleaguered security firm G4S "continue to rise significantly" but that ministers would "leave nothing to chance".
The 1,200 troops to be called up were placed on standby last week.
Mr Hunt said: "Security staffing levels at venues have been kept under constant review. G4S numbers continue to rise significantly and we have every expectation that will continue to be the case.
"However, ministers decided that we should deploy the additional 1,200 troops that were put on standby last week. On the eve of the largest peacetime event ever staged in this country, ministers are clear that we should leave nothing to chance.
"The Government continues to have every confidence that we will deliver a safe and secure Games."
Locog chief executive Paul Deighton said: "The reason that this decision has been taken is just to absolutely de-risk any aspect of the operation. With three days to go, we just want to make sure this works without any worries at all."
But he added that it was not a sign of private firm G4S having any further problems supplying staff. The extra troops come on top of the additional 3,500 drafted in a fortnight ago as the recruitment gap by G4S emerged.
Mr Hunt said the next 48 hours would be "critical" in terms of transport and security. "We are very confident that we will do a job but there is absolutely no room for complacency and the next couple of days are critical," he told a press conference, adding that the extra troops, which had been on 48 hours' notice, were called up "at the last moment" before the weekend.