Rural motorists could pay less under the scheme

But opponents say the government should not spy on people's movements.

Under the expected proposals, motorists using busy routes will pay more than on quieter roads, with vehicle movements being tracked by satellite.

If we don't plan ahead, over the next 20 to 30 years, we face almost certain gridlock Alistair Darling Transport Secretary

Mr Darling said he had to seek a radical approach to cope with rising volumes of traffic.

He told BBC Radio's Good Morning Scotland that congestion could be halved, with many people paying less than they do at present.

But he could not say how much a journey between Glasgow and Edinburgh would cost, although he claimed the �1.30-a-mile tariff would only apply to London.

Mr Darling added: "If we could just move a small proportion of people away from driving at rush hour you can free up a remarkable amount of roads.

"The difficulty we face, and in Scotland if people cast their minds back to 20 years ago, roads that were quiet are becoming busier and busier.

'Cheaper fuel'

"If we don't plan ahead, over the next 20 to 30 years, we face almost certain gridlock."

He said the technology could be introduced on a national basis in about 10 years time.

Terence Bendixson, secretary of the Independent Transport Commission (ITC) at the University of Southampton, believes the move would be good for Scotland.

The day the government put a microchip in my car that tells them exactly where I've been and when I went there, you can prise the keys of my Mercedes from my cold, dead hands Steve Berry Motoring journalist

He said: "All over the Highlands, all over rural Scotland, the rates are likely to be 2p a mile or something like that.

"That will give tremendous confidence and economic benefits for the Highlands, for the remoter parts of the country.

"At the same time, in the central belt, in Glasgow and Edinburgh, you'll have much less congestion."

Mr Bendixson added that removing the tax from fuel would see the price of petrol and diesel falling.

First Minister Jack McConnell said the Scottish Executive could "see merit" in charging, but that there was still a debate to take place.

Mr McConnell told MSPs at Holyrood: "For rural Scotland in particular this scheme could be a very good idea, but I want us to think through very, very carefully the impact on businesses and individuals in urban Scotland."

He indicated he would want assurances it was "revenue neutral" or provided a gain for motorists and said he would continue to engage with the department of transport in Westminster as the proposals develop.

Friends of the Earth Scotland gave the plan a cautious welcome.

'Welcome move'

Chief executive Duncan McLaren said: "Road building can't solve congestion, it simply generates more traffic.

"If this genuinely replaces building new roads like the M74 extension or the Aberdeen by-pass, then it would be welcome."

The Scottish Green Party also broadly welcomed road tolling, but said it must be targeted at reducing pollution and improving public transport.

Mr Darling hopes that road charging could reduce congestion

Transport spokesman Chris Ballance said: "Pay-as-you-go would only be effective alongside much bigger investment in a first class public transport system for all - revenue raised from road tolling should be ring-fenced for investment in public transport.

"We also need to encourage cleaner fuels."

However, motoring journalist Steve Berry attacked the invasion of privacy.

He said: "The day the government put a microchip in my car that tells them exactly where I've been and when I went there, you can prise the keys of my Mercedes from my cold, dead hands."

But Friends of the Earth said systems existed that could post-date the records so that the monitoring information was not live.

The Association of British Drivers called for better public transport to give car users a proper alternative.