More than 1.7 million people have signed a petition asking ministers to scrap plans for road charging. Tony Blair insists that the idea must be looked at as a way of tackling congestion.
Who wants road charging?
The government is a fan of charging schemes.
Although there are no tightly-defined plans, ministers have floated the possibility of replacing vehicle tax - and perhaps fuel duty - with a national pricing scheme.
The congestion sign is ubiquitous on London's roads
Labour's 2005 manifesto contained the pledge: "We will seek political consensus in tackling congestion, including examining the potential of moving away from the current system of motoring taxation towards a national system of road-pricing."
Ministers hope this will force motorists to consider car-sharing, using public transport or cutting down on unnecessary journeys.
The move towards road pricing has broad support from business and environmental groups - although that support comes with caveats.
The Confederation of British Industry says it backs charging "in principle" but stresses it "must be integrated with other policies including public transport and planning".
Similarly, campaign groups such as Transport 2000 and Friends of the Earth say ministers must improve public transport to make any road pricing scheme work.
The haulage industry is very much in favour of road pricing. It feels that foreign lorries have an unfair advantage because they are not taxed to use Britain's roads.
Who opposes charging?
There has been a groundswell of public opinion attacking plans to introduce a national system of road pricing.
This culminated in an e-petition on Downing Street's website, which was backed by more than 1.7 million people.
A separate scheme for lorries was planned, but ministers scrapped it
The petition was the idea of Peter Roberts, a 46-year-old account manager from Telford.
It read: "We the undersigned petition the prime minister to scrap the planned vehicle tracking and road pricing policy."
Mr Roberts is a member of the Association of British Drivers, which has called for a referendum on the issue of road pricing "as soon as possible".
The association is one of several lobby groups opposed to pricing - they include No To Tolls, which says road charging "favours the rich" and will need a huge expensive bureaucracy.
Some motoring groups are not opposed to changing the way drivers are charged, but they are not yet convinced a national pricing system is the answer.
The AA Motoring Trust said there was "huge uncertainty surrounding the likely costs and benefits of a scheme" and called for detailed studies to be undertaken.
The RAC Foundation has warned that the government needs to convince motorists money raised from the scheme will be used to improve roads.
How would a national scheme work?
Nobody knows. It is likely that a national system would involve the use of technology similar to satellite navigation.
Each car would be fitted with an electronic tag to monitor its road use and communicate the details via satellite.
Motorists would then be charged for each mile they drive.
Experts say other methods of collection - such as tolls or cameras - would only be suitable to combat congestion on a national scale in conjunction with a satellite system.
The government has not yet decided how a national system would work.
Ministers are funding pilot projects where local authorities will investigate how schemes can work on a small scale.
Dave Tindall, from the Transport Research Laboratory, has undertaken several research projects for the government. He believes a national scheme will "evolve" from these local pilot projects.
"The biggest technical challenge will be that, if every local authority comes up with its own scheme, how are you going to join them together?"
He compared the current state of technology with the Betamax and VHS video cassettes of the early 1980s - saying it would take time to work out which technology would work best.
Are any schemes operating at the moment?
There are many tolls throughout Britain on bridges, tunnels and the M6 motorway near Birmingham.
But there are only two fully-operational schemes aimed at tackling congestion.
In 2002, Durham became the first city to introduce a congestion charge. It controls access to its cathedral and castle area.
London's congestion charge, introduced in 2003, is the largest-scale charging system in the country.
The London scheme was heavily influence by Singapore, which implemented an electronic toll collection system.
Its Electronic Road Pricing scheme was introduced in 1998 and uses radio technology to deduct charges automatically from drivers' cash cards as they join restricted routes.
Germany and Austria have road pricing for lorries - each vehicle is fitted with a tag connecting it to a satellite.
Vehicles are charged between nine and 14 cents a kilometre - depending on their emission levels and number of axles.
Ministers had planned to introduce a similar scheme for lorries in Britain in 2008 but opted to wait until a national scheme for all vehicles can be introduced.
Would it cost more than current taxes?
In 2005, Transport Secretary Alistair Darling proposed charging motorists by the mile.
Under his plans, prices would start from 2p a mile on quiet roads outside rush hours, rising to £1.34 a mile on busy motorways like the M25 at peak times.
In 2003, the Institute of Public Policy Research think-tank conducted a study into the costs to the motorist of such a scheme.
It concluded that overall road pricing would have to raise more money than current taxes if it were to reduce congestion.
Under the scheme the institute envisaged, a low-mileage rural motorist in the north-west of England would save about £3 a month.
But an average suburban motorist in the south-east of England would pay about £32 more.
And a high-use business motorist based in the West Midlands might pay about £55 extra a month.