The M25 is Britain's busiest road

As a result, plans to widen parts of Britain's busiest road, the M25, could also be scrapped.

A study prepared for the government last year, said that charges should be brought in for roads within a 50-mile radius of London to solve congestion on the motorway.

But the South East England Regional Assembly said there were "serious concerns" about the charging plan.

It said Transport Secretary Alistair Darling should commission a study into the effects of such a scheme. Until then, the assembly said it reserved its position on area wide charging.

The assembly's view on the M25 study - known as Orbit - is important to Mr Darling, who is expected to make a final decision on the motorway's future later this year.

Traffic lights

Orbit also proposed that 50 miles of the 120-mile M25 should be widened to four lanes from the current three. But widening should only go ahead if road-user charges were introduced for roads within the 50-mile radius.

Orbit added that if such charges were ruled out, then the widening should only go ahead if motorists paid a 3p a mile toll to travel on the widened stretches.

Failing that, ramp metering - whereby the flow of vehicles coming on to the motorway is controlled by traffic lights - should be introduced.

The assembly's views will now be considered by Mr Darling.