The government's failure to set a budget for London's Olympic Games is a "major risk" to success, Whitehall's spending watchdog has warned.
The cost of the Olympic Park has jumped by £900m to £3.3bn
The National Audit Office (NAO) also said more public funding is likely to be required to meet rising costs.
The Public Accounts Committee, which the NAO reports to, said the "worrying" situation must be dealt with as a "matter of urgency".
The government said it will work with the NAO to ensure value for money.
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 2012 Games.
It said London council taxpayers and the National Lottery are already "at the limit" of what they can afford.
The original public funding package includes £2.375bn from the public sector and £1.044bn the government is providing towards the costs of infrastructure on the site of the Olympic Park in Stratford, east London.
The public funding package includes £1.5bn of Lottery funds, £625m from London council taxpayers and £250m from the London Development Agency.
The NAO report 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 £900m to £3.3bn.
An unforeseen VAT bill of about £250m has also pushed up the price of the Games.
There is also the cost of regenerating the Lower Lea Valley around the Olympic Park, estimated at more than £1bn.
A spokesman for the Department for Culture, Media and Sport (DCMS) said: "We have been engaged in a rigorous review of costs since we were awarded the Games in July 2005 and Tessa Jowell has invited the NAO to work with DCMS to scrutinise the 2012 budgets.
"We look forward to co-operating with them to ensure value for money for the public."
Meanwhile, lottery games have raised £100m for the London's 2012 Olympic Games, it has been announced.
The Olympic Lottery Distributor now believes it will raise £118m in income by March 2007, towards the £1.5bn, instead of the £110m as originally planned.