The charge, which could start at £5 to £10 in 2008, would be the first mandatory levy on drivers using Britain’s motorway network. It would rise to about £20 in 2015 to ensure that stringent European emissions targets are met, clearing one of the hurdles for the construction of a third runway at Heathrow.

The proposals are outlined in an internal memo drawn up by transport department officials and published under the Freedom of Information Act. If approved, the charge will affect up to 150,000 motorists who use the M4 each day to enter and leave London. The fee would be imposed on top of the capital’s £8 congestion charge.

The toll could be enforced by cameras similar to those already used in central London or satellite-based tracking devices. Higher charges could be levied at peak times, while the most polluting vehicles might be banned altogether, according to the document.

Edmund King, executive director of the RAC Foundation, the motoring body, described the proposals as “one-sided”. He added: “If you did have a charge on the M4 just because of access to Heathrow, it would hit many millions of other motorists who have no intention of catching an aeroplane.”

Critics believe that traffic could be displaced to smaller roads — creating new congestion hot spots — as drivers tried to avoid the new charge.

However, the document predicts that people will abandon their cars: “The increased price of using the road network would encourage substitution away from the road network towards public transport.

“This has two impacts: it reduces the congestion on the road network, thus generating user benefits for those individuals who still use the roads; and it has the knock-on effect of reducing the level of emissions. The LAQ (local air quality) problems associated with Heathrow mean that this achievement is particularly worthwhile.”

The government favours the construction of a new runway at the west London airport between 2015 and 2020, which would increase passenger numbers from 67m to 116m by 2030. However, it will be approved only if air pollution around Heathrow can be tackled first.

European legislation will make it illegal from 2010 for residents to be exposed to nitrogen dioxide emissions of more than 40 microgrammes per cubic metre of air.

Although most of these emissions are produced by aircraft landing and taking off, officials claim that traffic from the nearby M4 and from a spur road linking the motorway to the airport is also to blame.

The document refers to Highways Agency research which has examined the impact on traffic levels of both the opening of Terminal 5 (T5) in 2008 and eventually a third runway. This, in turn, was based on an earlier study which proposed road pricing along the wider M4 corridor to the west of London.

“A charge in the range of £5-£10 on the M4 within the M25 and M4 spur would be significant enough to avoid the need for engineering work (such as adding an extra lane) on the M4 to account for new T5 traffic alone,” the document says.

“Modelling work . . . suggested that a charge in the region of £20 would be required to restrict traffic to T5 levels if a third runway was constructed.”

A spokesman for the transport department said there were no immediate plans to introduce tolls on the M4.

However, Alistair Darling, the transport secretary, will today reiterate the need to consider a national road pricing scheme when he addresses the Labour party conference in Brighton. He will say that tackling congestion is crucial if Britain is to compete with emerging economies such as China and India. Nick Ray

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