The Public Accounts Committee said in a report published on Friday that Olympic budgets are “finely balanced”.
The committee detailed its concerns over security budgets and the amount of public money being controlled by the London Organising Committee of the Olympic Games – a private company.
The PAC said Locog’s original estimates of venue security costs were based on a “finger in the air estimate”.
Locog has more than doubled its security allocation from 10,000 to 23,700 –meaning the cost has risen from £282 million to £553 million.
Locog has £5 million headroom in its own budget, said the report, meaning that the government may be liable if it overspends.
Ten days ago, Olympics Minister Hugh Robertson said the Games were “likely to be under budget”.
But the chair of the PAC, Margaret Hodge, said: “It is staggering that the original estimates were so wrong…there is a big question mark over whether it secured a good deal for the taxpayer.
“Taking into account costs outside the package, the full cost to the public of the Games and legacy projects is already heading for around £11bn.”
Costs are predicted to rise once the £766 million cost of buying back the Olympic Park and £826 million legacy costs are accounted for.