THE road-charging revolution came a step closer last night when Transport Secretary Alistair Darling announced plans to counter the threat of national gridlock.

The move sparked fury among road-user groups and brought claims that the system would be used as a "spy in the sky".

He confirmed he was looking into introducing a road-pricing scheme that would charge motorists for the journeys they made instead of paying road and fuel taxes.

Satellite technology similar to GPS road navigation systems would track every mile driven through a black box fitted to every vehicle.

Motorists would be charged according to the time of day and congestion of the roads they used.

Mr Darling said: "I believe that, in this parliament, we have got to decide whether or not road pricing is feasible, whether it's the right thing to do, because if we don't do anything then we will face gridlock in 20 to 30 years' time."

He said he would be looking for an area of the country to pilot the scheme, after a study found it could cut congestion by almost half.

But as road groups reacted with anger to reports that motorists could be charged as much as £1.34 a mile for using congested motorways, the Transport Secretary distanced himself from any figures, saying it was impossible to put a price on anything at this stage.

He said: "I'm not in the business of trying to stop people driving, or making it so prohibitively expensive that it becomes a luxury for just a few.

"What I am talking about is a sensible charge so that you can make reasonable decisions about where and when you travel."

But Mike Rutherford, from the Motorist's Association, attacked Mr Darling's proposal as "a money-making racket".

"It's almost legalised theft," he said. We already pay £40bn into the Treasury. If Alistair Darling thinks it's OK to charge £1.34 a mile for travelling on the motorway, then he is living on an even more distant planet than I thought he was."

The scheme would not be introduced for at least a decade, but a feasibility study carried out last year suggested that charges could range from 2p a mile on rural roads to £1.34 a mile for peak-time journeys on the country's busiest roads and motorways.

Mr Darling, who will set out his thinking in a speech on Thursday to the Social Market Foundation, acknowledged that he would need to build a consensus over the course of this parliament if the scheme was to win public acceptance.

He emphasised the new charges would replace fuel tax - dramatically cutting the price of petrol - and also, possibly, road tax.

The Conservatives and Liberal Democrats dubbed the idea the "spy in the sky" proposal and said it must not be another "stealth tax".

While the Lib Dems said it would be no substitute for an integrated public transport system, the Tories said there were question marks over the technology surrounding the scheme.