The "chaos" surrounding the failure of G4S to provide enough staff for the Olympics has undermined confidence in Games organisers, a committee of MPs has said.
The Public Accounts Committee (PAC) is calling for G4S to face financial penalties over the "last-minute scramble" which has seen 3,500 extra soldiers drafted in to cover the shortfall.
The committee's chairman Margaret Hodge said the Home Office should "get a grip" on G4S and games organiser Locog.
On Tuesday, G4S chief executive Nick Buckles told the Commons Home Affairs Select Committee he could not deny the debacle was a "humiliating shambles for the company".
PAC chairman Hodge has also criticised the Department for Culture, Media and Sport (DCMS) for a lack of transparency over Games finances.
Ms Hodge said: "The chaos which has emerged over the security contract was predictable and undermines confidence in those responsible for managing the Games.
"There is now a last-minute scramble to make sure there will be enough security personnel on the ground.
"In evidence to us before Christmas, all those concerned gave assurances that the contract could be met despite more than doubling the number of guards before the ink was dry on the original contract.
"No credible explanation has been given for an astonishing 12-fold hike in management costs, from £10 million to £125 million, and G4S still has not been able to deliver.
"Now troops are having to be drafted in. The Home Office needs to get a grip on Locog and G4S urgently.
PAC chairman Hodge has slammed the Games organisation as well as Department for Culture, Media and Sport (DCMS) for a lack of transparency over London 2012 finances.
"The department would win no medals for transparency. We still don't know just how many security guards will actually be delivered or whether G4S will face any penalty. We will carry out a post-mortem in September."
The report states that G4S should not only pay for all additional costs but also face financial penalties for the "failure to deliver".
The committee's report says there has been no overall figure about how much the Games are costing the taxpayer.
There is a public sector funding package of £9.3 billion but there is other Games-related expenditure, such as funding of elite athletes, which has also come from the public purse.
Ms Hodge added: "We're not happy with how the DCMS (Department for Culture Media and Sport) has engaged with this committee.
"Legitimate monitoring of implementation has been an unnecessarily frustrating process, characterised by a resistance to giving us clear and consistent information and a readiness to depart from proper ways of conducting public business.
"We have faced considerable difficulty in pinning down just how much the Games is costing the taxpayer and how much risk there is.
"A big concern is that the department has no intention of producing a single auditable account for the Games, drawing together both the costs within the public sector funding package of £9.3 billion and those outside. Such an analysis must be produced."
Olympics Secretary Jeremy Hunt said: "With just over a week to go, we are on track to deliver a great Games on time and under budget.
"At the end of May there was £476 million of contingency unallocated and I am confident that we will deliver the project for under £9 billion.
"We disagree with the Committee's assertion that the Department for Culture, Media and Sport has lacked transparency. London 2012 has arguably been more transparent than any other public sector project.
"For example, we have produced 16 financial updates to Parliament over the last four years. The ODA set out public milestones for delivery from the outset against which success could be measured and we have engaged with the Committee at every stage over the last seven years.
Labour leader Ed Miliband wants G4S to be barred from taking on any more government contracts until its competence has been reviewed in the wake of the Olympic security scandal.
In a speech on Thursday Miliband will demand a moratorium on any new public money being awarded to the giant firm after what he said was a failure that "beggared belief".
And he will present the fiasco as evidence that the coalition should halt moves to massively increase private sector involvement in policing, warning it will undermine forces' resilience.
The under-fire security giant is already involved in £600 million worth of private work for the Home Office alone and had been hoping to further increase its share of the market.
But Mr Miliband, addressing a gathering of his party's candidates to be the first elected police and crime commissioners (PCCs) in England and Wales, will say that ambition must be put on hold. "Before they are awarded any new policing contracts, we need a review of G4S's ability to deliver," he will tell them.
His call comes amid reports that a request may be made for up to 2,000 additional troops to plug any further gaps in security if G4S's problems become more severe.
The Ministry of Defence, which has already upped its contribution by 3,500 to 11,000, said that contingency plans were in place to increase the numbers again if necessary.
David Cameron pledged on Wednesday to "go after" G4S over the Olympics security chaos as the government targeted the firm's multimillion-pound management fee.
G4S boss Nick Buckles made an humiliating appearance before MPs on Tuesday during which he admitted that there would be financial consequences from the company's failure to recruit enough staff but insisted it wanted to keep a £57 million management charge.
However sports minister Hugh Robertson said the government was activating "all penalty clauses" in the contract, some of which applied to the management fee.
Both Mr Cameron and Mr Miliband have been keen to stress that they fully expect the Games to be a major success despite the problems as the row continues to overshadow the final build-up.