Having no agreed final budget or funding package 18 months after winning the right to host the Games is proving a 'major risk' to success, Whitehall's spending watchdog said.
Edward Leigh, chairman of the Public Accounts Committee, said this 'worrying' situation must be dealt with as a 'matter of urgency' if plans to build a lasting legacy are to stand a chance.
Last week the Culture, Media and Sport Committee called on the Treasury to 'put its hand in its pocket' to meet the soaring costs of the Games.
It said London council taxpayers and the National Lottery, widely seen as the obvious choices to meet extra financial demands, are already 'at the limit' of what they can afford.
The NAO report into the financial management of the Games concludes: 'A major risk is the lack of final agreed cost estimates and an accompanying funding package, and this will inevitably have a detrimental impact on the programme if it is allowed to continue.'
Olympics Minister Tessa Jowell has admitted the cost of the Olympic Park has jumped by £900 million to £3.3 billion.