The Olympic Stadium is estimated to cost £525m
Olympics Minister Tessa Jowell has told the BBC that there will be no extra money beyond the £9.325bn already allocated to the 2012 London Games.
Any overspends on Olympic projects would be balanced by scaling back spending in other areas, she said.
Earlier London mayor Boris Johnson said: "We will come in on budget and we will not waste taxpayers' money."
At the time of the bid, costs were estimated at some £2.4bn, but last year the budget was put at £9.325bn.
And the National Audit Office has warned that spending may rise further due to uncertainty of costs of security and unsigned construction contracts.
'No more money'
But Ms Jowell told the BBC earlier: "Within the overall ceiling of £9.325bn there's no more money."
Asked if she was confident the project could be completed within that budget, she added: "The budget can not be exceeded because there is no more money.
"If we have to find more money for a particular aspect of the programme, then savings will have to be effected elsewhere and everybody involved in the programme understands that."
BBC sports editor Mihir Bose said there were already suggestions the athletes' village would be scaled back and security costs might rise.
Meanwhile he said British athletes face a potential £100m shortfall in sports funding, which UK Sport had expected to come from the private sector but is finding difficult to raise because of the credit crunch.
A further £500m is coming from the government and National Lottery.
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There is no doubt that funding makes things easier, as does self belief
F Fuller, Tunbridge Wells, UK
The chairman of the British Olympic Association, Lord Moynihan, has called on Prime Minister Gordon Brown to commit the government to make up any shortfall.
Mr Johnson acknowledged that China had put on a "fantastic show" but London would "put on something just as fine".
He told BBC Radio 4's World at One programme that it was "absolutely right" that no more money should be added to the budget but said organisers would be looking at where savings could be made.
He said: "This was a project, an Olympic Games, that was won, secured, commissioned at a time of economic plenty.
"We're being asked to deliver it in a credit crunch and with what people say is a recession looming. The International Olympic Committee understands that.
"We're not going to produce an austerity games, we are not going to run the thing down but we are going to deliver value for money and I'm absolutely determined that we are not going to waste taxpayers' money.
"And that means that over the next few months we will be looking at ways of ensuring we maximise bang for our buck.
"Does every single thing on the site have to be on the site, that's the kind of question we'll be looking at."
Earlier the Conservative former prime minister Sir John Major said all political parties should back guaranteed, long-term National Lottery funding for grassroots sport, from where the Olympic stars of the future would come.