Olympics security will not be compromised, Home Secretary Theresa May has said, after it emerged 3,500 extra troops would be needed.
She said it was only discovered on Wednesday - 16 days before the Games begin - that contractor G4S did not have enough trained security staff.
The troops are in addition to 7,500 already agreed for venue security.
Labour MP Keith Vaz said: "G4S has let the country down and we have literally had to send in the troops."
G4S was contracted by the London 2012 Organising Committee to supply 13,000 staff.
It said it had 4,000 people already working across 100 venues, with a further 9,000 going through the final stages of extensive training, vetting and accreditation.
"We have encountered some delays in progressing applicants through the final stages but we are working extremely hard to process these as swiftly as possible," the company said.
Mrs May addressed the Commons in response to an urgent question from Mr Vaz.
"Can the home secretary give the House this assurance: that she is now satisfied that all the changes she's announced today will mean that this, what we hope will be the greatest Games ever staged, will be done securely for the safety of the visitors and the British people?," he said.
'Most important' task
Mrs May responded: "I can confirm to the House that there remains no specific security threat to the Games and the threat level remains unchanged.
"And let me reiterate that there is no question of Olympic security being compromised."
Mrs May said Britain had the "finest military personnel in the world".
"They stand ready to do their duty whatever the nation may ask. Our troops are highly skilled and highly trained and this task is the most important facing our nation today," she said.
Shadow home secretary Yvette Cooper said the scale of the problem should have been recognised sooner.
She welcomed the decision to bring in extra troops, but added: "This really looks like another huge Home Office shambles."
In a statement, Downing Street warned the company should face consequences for its failure to fulfil its contract - for which it is being paid £284m.
The London 2012 Organising Committee (Locog) said it was not commenting on contractual arrangements with G4S.
Defence Secretary Phillip Hammond said he had authorised the deployment of the 3,500 military personnel, bringing the total number of military personnel - from three services and including reservists - contributing to Games security to 17,000.
Personnel had already begun to be deployed to venues "to support the rolling search and lock-down process between now and the start of the Olympics, alongside the police, the commercial security provider, G4S, and volunteers," he said in his written ministerial statement to Parliament.
Military personnel will carry out general security on venue gates and bag checks. Some 5,000 personnel will be in specialist roles, such as bomb disposal squads, special forces and the London missile sites, with a further 1,000 involved in logistical support.
Mr Hammond said the Chiefs of Staff supported the deployment, adding it would not have an adverse impact on other operations.
"We will ensure that all those taking part receive their full leave entitlement, even if it has to be rescheduled, that no-one is out of pocket due to cancelled personal arrangements and that all deployed personnel are appropriately supported," he said.
Mrs May gave further details, saying 10,000 Olympic and Paralympic tickets had been donated to the armed services via Tickets for Troops, there was access for 2,000 to spectator areas in the Mall for the Olympic cycle road races and Olympic marathon, and troops had the right to buy 2,000 Olympic Park tickets.
Also, 7,000 tickets had been offered to the troops for the dress rehearsals of the opening and closing ceremonies.
Mr Hammond later told the defence select committee that the deployment request had come "as no great surprise".
He told MPs it became clear that some extra troops would be needed two weeks ago when the beginning of the lock-down at the park started.
He said the troops had their "notice to move" reduced at the weekend, which informed them of imminent deployment. All of the extra troops had been on standby for such a contingency.
The news of extra troops came ahead of the chief inspector of borders raising fresh concerns about the border controls at Heathrow in the run-up to the Olympics.
Chief Inspector of Borders John Vine said some staff were not dealing with arriving passengers efficiently or effectively enough and that forged documents could be going undetected.
Downing Street said G4S's inability to provide enough security for the Olympics was "unfortunate".
The prime minister's spokesman said a joint military-civilian security operation was always envisaged at the Olympics and "we are happy to change that balance."
The spokesman said the government would meet the cost of the redeployment of the troops but refused to say whether Locog would force G4S to recompense the taxpayer.
Mrs May said the government remained confident the venue security costs would remain within the £553m budget.
One woman, Catherine Roberts, said she was now unemployed after being turned down for a position with G4S in Cardiff, despite being given two weeks of training in London in June.
She said although she was told she had passed all the necessary tests and training, her employment was terminated - first because she was told he had failed a course and then because of incorrect documentation.
"The money they have wasted on sending me to London and through courses that cost £200 to then lay me off is ridiculous. I am absolutely gutted as I have been left jobless with no way of paying for my rent."
The company was also one of four contractors bidding to take over some services for Surrey Police, but the region's police authority voted on Thursday to suspend the plan - and is now thinking of scrapping it entirely.
Members of Surrey Police Authority were unhappy about G4S's failure to recruit enough guards for the Olympics.