The total cost of the 2012 Olympics may rise above the government's "final" revised estimate.
The National Audit Office (NAO) warns many "significant uncertainties" underpin the Olympic budget, announced by Tessa Jowell in March, and costs may rise above £10 billion by 2012.
Ms Jowell, then culture secretary, announced a £9.3 billion budget for the Games in March, significantly increasing the £2.375 billion estimate put forward in 2003.
The NAO said the inflated figure would be sufficient to cover the cost of the project as currently planned. But this will only remain the case if the assumptions on which the budget is based hold good.
Auditor general sir John Bourn said: "The Olympic Games is now on a firmer financial footing thanks to the budget announced in March 2007.
"This should help all those involved in delivering the Games to move forward with greater confidence. However a budget is just that - a budget, not a target."
An investigation by the NAO found a number of areas where costs are not certain, including the venue design, construction price inflation and private-sector funding could affect prices.
The Department for Culture, Media and Sport was also urged to keep a close watch on the original £2.7 billion contingency allowance.
Ms Jowell, now Olympics minister, said the government would keep an "iron grip" on costs.
"With a robust funding package in place and world-class team at the Olympic Delivery Authority, we are now in an excellent position as we move into the next phase of the project," she said.
The public accounts committee (PAC) has already raised concerns the designs for all Olympic sites have not been finalised.
Chairman of the committee Edward Leigh said the terror threat and associated security costs could also push the bill higher.
Mr Leigh estimated the final bill could surpass £10 billion, a quadrupling of the original estimate.