Drivers already pay to use the Dartford crossing, linking the north and south sections of the M25, and ministers are understood to believe an extension for charges to the whole of the motorway would be feasible.
It is deemed so politically sensitive it is unlikely to be introduced until after the next election but its backers believe it is inevitable. More than pounds 200m a year could be raised, a sum Treasury officials believe could be used to improve public transport in London and on the motorway.
Car drivers using the Dartford crossing pay pounds 1 and lorries pounds 2.70, but legislation is needed to extend the charging regime.
Officials at Mr Prescott's Department of the Environment, Transport and the Regions are looking at ways of extending the levy to cover the whole motorway, probably by introducing cameras that would recognise vehicles with a smart card.
News of the scheme follows publication this week of Mr Prescott's consultation paper on giving councils powers to impose charges in their areas. The paper also disclosed that the proposed legislation would allow charging on trunk roads and motorways. A Bill to allow pilot projects could come in the current session of Parliament if Tory peers abandon their campaign to disrupt all government business.
Road-pricing powers are also included in the Greater London Bill for a directly elected mayor and assembly for the capital.
The only motorway so far to be earmarked for tolls is the Birmingham northern relief road, a public-private partnership inherited from the last government.
A spokesman for Mr Prescott's department said no decisions had been made.
Chris Fisher, of Fight Link Roads and M25 Expansion (Flame) said the plans were aimed at funding extra lanes for the motorway.
"People to the west of London will not only be subjected to more pollution but asked to pay for it," he said.
"The M25 road-widening scheme is nothing to do with easing traffic congestion and everything to do with providing BAA with a super-highway access to Terminal Five. The fat cats of BAA should be asked to fund it, not the motorist."Reuse content