Two Kent stretches of the M25 are to be widened to four lanes

The motorway will be expanded to four lanes each way between the Dartford Crossing and the M20.

The stretch from the junction with the M26 at Sevenoaks and the M23 in Surrey will also be widened.

And Mr Darling also revealed plans for a faster train service between Ashford in Kent and Brighton in East Sussex.

Five sections of the M25 are to be widened at a total cost of �1.7bn following the recommendations made in the Orbit (M25) Multi-Modal Study.

Widening roads will not solve congestion: it will simply put more cars on the road and make things worse in the long run Stephen Joseph, Transport 2000

They include the areas between junctions 1b (the Dartford Crossing over the Thames) and three (the junction with the M20 at Swanley); and between junctions five (where the M25 meets the M26 at Sevenoaks) and seven (the junction with the M23 in Surrey.

There will also be improvements to junction five.

A faster train service will run eastwards from Ashford, providing an hourly express service to Brighton, via other Sussex towns including Hastings, Eastbourne and Lewes.

Green transport group Transport 2000 condemned the plans to widen the M25.

Director Stephen Joseph said: "Widening roads will not solve congestion: it will simply put more cars on the road and make things worse in the long run.

"It will be bad for motorists and bad for the environment.

"The M25 Orbit Study itself said that widening alone will simply lead to bigger, wider traffic jams."

He said many environmentally sensitive sites would be affected by the road schemes set to go ahead and that many people would face increased traffic noise and air pollution.