More than 18,000 troops will provide security for the Olympics after ministers felt they should "leave nothing to chance".
Culture Secretary Jeremy Hunt said the 1,200 servicemen and women put on 48 hours notice last week will be deployed in the wake of the Olympics security shambles.
With the opening ceremony three days away and the football starting on Wednesday, organisers Locog said the move would bring in enough troops to deliver the Games "in just about any scenario".
The extra military involvement comes on top of the additional 3,500 troops drafted in a fortnight ago as the "humiliating shambles" created by private firm G4S's failure to supply enough guards emerged. A further contingency force of 1,000 troops is ready to step in if there is any increase in the security threat level. Ministers took the decision at a Cabinet committee for the Olympics chaired by Prime Minister David Cameron.
Mr Hunt added 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 said.
Amid concerns over a lack of training for staff in how to use the airport-style X-ray machines and other security measures, roving teams will be brought in to check guards' work and make sure it is up to scratch, Locog said.
Mr Hunt said: "On the eve of the largest peacetime event ever staged in this country, ministers are clear that we should leave nothing to chance. 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. The Government continues to have every confidence that we will deliver a safe and secure Games." He added the additional deployment was not because of any deterioration in the performance of G4S.
Locog chief executive Paul Deighton added: "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."
He also denied it was a sign of further problems at G4S, saying just under 6,000 of the 10,400 guards the firm was contracted to supply have been delivered so far. Mr Deighton went on: "You can't be absolutely certain of anything with a temporary workforce. Therefore we want to substitute a temporary workforce with a permanent, reliable workforce that we get with the military. We now have the full military deployment to deliver these Games in just about any scenario, so I'm confident about that."