The US will this week thwart European plans to force airlines to pay for pollution from flights.
The move will be a blow to Tony Blair, who backed the aviation charging proposals earlier this month.
The challenge will come from the International Civil Aviation Organisation (ICAO), made up of senior transport officials from around the world. US members of the ICAO have tabled a resolution that, if successful, would block European member states from introducing emissions charging. The resolution will be considered at the ICAO's conference in Montreal, which begins on Tuesday.
Insiders said the resolution would almost certainly be passed because the US had secured the support of several nations, including Japan, Saudi Arabia and Brazil.
Proposals for European emissions charging are being developed beside an emissions trading scheme, where "green" airlines can sell credits to ones that produce higher levels of pollution. Britain and the EU have argued that both schemes need to be set up in tandem.
A spokesman for the UK's Department for Transport said: "The ICAO should not close off options such as market-based measures. The UK preference is for emissions trading, with charges only as a fallback or possible supplement to trading."
Emissions trading is supported by British Airways, which is taking part in a UK trial of the system.
If ICAO members vote to block emissions charging, the decision is not legally binding. But sources close to the European Commission said that rejecting an ICAO resolution would be "unprecedented".
The US is understood to have tabled the amendment because it was concerned that its airlines would be "caught" by European emissions charging. One well-placed source said: "The US is suspicious that the scheme would extend half- way across the Atlantic."
The US stance has disappointed environmentalists. Tim Johnson, the director of the Aviation Environment Federation, said: "The rate of aviation growth is outstripping technological innovation in efficiency. The UK Government should make a stand so that emissions charging is not closed off."
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