The government is pushing ahead with plans to introduce road pricing schemes in England and Wales despite a huge public campaign against them.
The government says schemes could tackle rising congestion
It has published a draft bill updating the rules for local authorities who want to set up charging trials.
It insists there are no plans yet for a national scheme but critics say it is not being open about its intentions.
A petition against road pricing on the Downing Street website received nearly two million online signatories.
Widespread road pricing is at least 10 years away technically - but 10 local authorities have expressed an interest in developing smaller-scale charging systems in their areas, which could be up and running within five years.
The draft Local Transport Bill will give councils more flexibility to match road pricing schemes to local conditions, while ensuring they remain compatible with schemes in other areas.
KEY POINTS OF DRAFT BILL
Update existing powers so councils can propose local road pricing schemes
Any scheme expected to be part of anti-congestion plan and to fit in with those run elsewhere
Councils to bring in "quality contracts" to specify companies' bus routes, timetables and fares
Reform passenger transport authorities in major towns outside London to enable a "more coherent" approach
But a Department for Transport spokesman said this did not mean the government was pressing ahead with a national pay-as-you-drive scheme.
"No decision has been made on a national scheme. We have got to see the results of the pilot schemes," he said.
He said there would be a three-month consultation period for those in favour and against road pricing to have their say before a final bill is drawn up.
Prime Minister Tony Blair, in his February reply to the Downing Street website petition, also insisted no decision had been made on national road pricing.
But he said congestion could not be allowed to grow unchecked and any scheme would not be used as a "stealth tax".
Mr Blair's official spokesman said earlier that the Downing Street website petition had "revealed the strength of feeling" on road pricing but "that was not a substitute for policy".
He said no decision had been made on a national scheme, but added: "If you don't have anything congestion will increase."
Conservative transport spokesman Chris Grayling said: "If the rumours about the bill are right, it is clear that Gordon Brown has his eye on the revenue a national scheme would bring in.
AREAS BIDDING FOR PILOTS
Greater Bristol area
Shropshire for Shrewsbury
Tyne & Wear
Nottingham, Leicester and Derby and the surrounding counties
"This Bill, from Douglas Alexander, one of his key lieutenants, shows Brown's determination to introduce national road pricing is just as strong as Tony Blair's was."
Liberal Democrat transport spokesman Alistair Carmichael said: "The government must be open and honest with people about its intentions to push forward with road pricing.
"They must commit to a system which does not mean motorists as a whole paying more, but just paying differently.
"If the public feel that road user pricing is just another cash cow for the Treasury, then it will meet stiff resistance and a real opportunity to reduce congestion will be missed."
The Federation of Small Businesses (FSB) called on the government to hold a referendum in areas where it was being planned.
"Without it road charging is completely illegitimate, as it would be for a national scheme without a public vote on the issue," said Steve Collie, FSB Transport Chairman.
"Instead of creating more tolls and more laws the government should be enforcing current rules and spending more on the transport network."
The main thrust of the draft Local Transport Bill concerns bus services, with local authorities given the opportunity to insist private operators run certain routes.
In return, councils will have to provide the appropriate bus lanes.
In London, where bus services are privatised but are still regulated, there has been an increase in passenger numbers in recent years.
But outside the capital, where services have been privatised and deregulated since 1986, there has been a fall in use.
The draft bill will fall short of proposing re-regulation for non-London services but it will give local councils - especially the big metropolitan authorities - more say in the running of buses.
In February, 74% of the 1,006 people questioned for a BBC-commissioned survey said they were opposed to charging motorists by the mile.
But 55% of those spoken to said they would change their minds and support such a scheme if the money raised was used to improve public transport - a move included in Tuesday's draft Bill.