The charge would affect thousands of motorists entering a fixed cordon around Heathrow, possibly including the main spur road that links it to the M4 motorway. Ministers hope that the move will encourage more people to travel to the airport by public transport, with money from the new levy likely to be used to upgrade existing rail and bus facilities.

Without this shift in travel patterns, the government fears that it will be unable to meet a stringent European Union emissions cap that comes into effect in 2010 and has the potential to delay plans to build a third runway at Heathrow.

Proposals for the congestion charge are outlined in documents released by the Department for Transport last week under the Freedom of Information Act. The documents relate to the meetings of a stakeholder group examining access to the west London airport.

They reveal how the group — which includes officials from the transport department and executives at the British Airports Authority, the owner of Heathrow — has commissioned Sinclair Knight Merz, a leading consultancy, to forecast the impact of different charges on traffic levels.

Although Alistair Darling, the transport secretary, has backed a national road pricing scheme, the satellite-based technology needed for such an ambitious project would not be available until 2015 at the earliest, according to a government feasibility study.

However, a localised scheme — similar to that already operating in central London — would be possible to implement much sooner at Heathrow.

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