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 tomorrow, 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 this morning 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.
"With three days left to the opening ceremony and an incredibly busy weekend we didn't want to leave anything to chance, and we just decided this was 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."
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."
Both the military and G4S staff would also receive extra on the job training as a result of the debacle, Mr Deighton said.
"What we're particularly doing to try and reinforce performance on the ground is we'll have a series of roving teams go around and train people on the job to make sure they're up to scratch and performing," he said.
"So it's not a question of just training them and sending them out there."
Mr Deighton added that G4S's failure had been a "big disappointment", but insisted the Locog contract was "very, very strong" and the problems came from G4S's "poor performance".
"The plan is absolutely 100% in place - we simply have soldiers doing work which otherwise would have been done by private security guards," he said.
"Net outcome - we're actually in a better position."
Metropolitan Police Assistant Commissioner Chris Allison, the national Olympic security co-ordinator, said the troops offered extra flexibility.
"We don't know what's going to come round the corner, and that's the reality," he said.
The security operation in place was designed on the assumption that the threat to the UK from international terrorism would be severe during the Games, but it remains one level below that at substantial.
"We're doing the right things to thwart the terrorists," Mr Allison said.
"We've looked at the way in which terrorists have attacked in the past and we've tried to make sure that none of those can get through our security measures.
"I think we're in a very good place."
He added that on peak days there would be 12,500 officers on Olympics duty across the country, including 9,500 in London.
To help boost police numbers, all training has been cancelled, the number of days off for officers has been reduced and the proportion of officers who can be on holiday has also been cut, he added.
Shadow home secretary Yvette Cooper said: "The priority now is for everyone to pull together to ensure this is a great sporting spectacle.
"After enjoying what I hope will be a brilliant Games, the public will rightly expect the Government to return to Parliament in September to answer important questions about why G4S was allowed to fail, the deployment of these additional troops at this point as well as the 3,500 already deployed, and the information given by the Home Secretary to Parliament."