The US Treasury Secretary Timothy Geithner has told Sky News he is "not prepared" to support Gordon Brown's call for a financial transaction levy.
Earlier, the Prime Minister called on G20 leaders to consider a tax on financial transactions to protect taxpayers and make banks more accountable.
The money collected could be held in a global fund to bail out failed banks, Mr Brown suggested.
But Mr Geithner's rejection of the proposal will deal a fatal
blow to the Government's ambitions.
In an exclusive interview, he told Jeff Randall Mr Brown's plan is "not something that we're prepared to support".
He said: "I think we all share a basic interest in trying to make sure we build a system where taxpayers aren't exposed in the future and where the financiers are bearing the consequences of their mistakes - that they are responsible for the risks they take."
Mr Brown said the plan would only work if adopted globally and Chancellor Alistair Darling told the G20 summit that the International Monetary Fund, which oversees the global financial system, is considering it.
But Mr Geithner explained the US had backed a different method for ensuring the accountability of banks.
He said: "Now we proposed in the United States a way to achieve that, by making sure that if ... the government is exposed to any risk of loss, that we recoup that loss by assessing a fee on the liabilities of banks.
"Now we think that approach helps reduce the risk of moral hazard, it's fair to the tax payer and it doesn't put us in the position where retail investors and pension funds are the ones bearing the burdens of that cost."
Mr Geithner is in Britain for the latest G20 summit, being held in St Andrews, Scotland.
Hosting the talks, Mr Darling earlier urged the world's top finance leaders to reach a deal on bearing the cost of climate change before the UN summit on global warming in Copenhagen next month.
He also said leaders must agree a "framework for future growth" at the summit, which will involve finding agreement over financial stimulus plans.
Mr Geithner said he backed the Government's ambitions for continued fiscal support for struggling economies, despite earlier rumours the US would aim for a quick exit strategy.
He told a news conference: "It is too early to start to lean against recovery.
"The classic mistake in past crises was to put on the brakes too quickly.
"But we all recognize that confidence in our ability to reduce future deficits and to exit from the extraordinary monetary policy and financial emergency measures is very important to confidence in the sustainability of recovery."