Thousands more soldiers than originally planned are being drafted into Olympic security arrangements after the private firm responsible was forced to request help.
Home secretary Theresa May faced questions about her judgement in the Commons after she admitted that contractor G4S had failed to recruit enough personnel for the job.
There are supposed to be 23,700 security guards at the Olympic Games, which begin in a fortnight.
The armed forces were supposed to be providing 13,500 but that has now been revised upwards to 17,000, May confirmed to MPs.
"It would be prudent to provide greater military support to provide greater reassurance," the home secretary conceded.
Leave has been cancelled but more Olympic tickets have been provided for troops and those forced to cancel summer holidays will not be left "out of pocket", May pledged.
Home affairs committee chair Keith Vaz, who secured the urgent question that forced May to the Commons chamber, pointed out that the home secretary had reassured MPs as recently as Monday that all was well.
May had told the House that she was "confident that our partners will deliver a safe and secure Games".
"G4S has let the country down and we have literally had to send in the troops," Vaz declared.
The home secretary said the government had been received reassurances from G4S "until very recently" and that the precise numbers had only "crystallised" yesterday.
That did not convince shadow home secretary Yvette Cooper, however, who told MPs: "This looks like another Home Office shambles."
She demanded to know why it took so long to establish that G4S had committed a breach of contract of approximately 25%.
"Everyone is working really hard to make the Games a success and show the world the best of British," Cooper said.
"The Home Office is making that harder, not easier, so I say to the home secretary, please – get these… problems sorted out and stop letting everybody else down."
May replied: "It is not a shambles when the government takes the action necessary to make sure we are providing security."