Road pricing cameras could be used by police to track drivers' movements in England and Wales under new proposals.
Drivers in London will be visible in real time to anti-terror officers
A Home Office document accidentally released suggests police should be given instant access to cameras which monitor congestion and road charging.
It comes after anti-terror officers were given real time access to congestion charge cameras in London.
The Lib Dems accused the government of using that announcement as a "Trojan Horse" for more wide-ranging proposals.
The party's home affairs spokesman Nick Clegg said: "With this unintended act of open government the disingenuous attitude of ministers towards public fears about a creeping surveillance state is revealed for all to see.
"Bit by bit vast computer databases are being made inter-operable and yet the government seems to be running scared of a full and public debate on the safeguards needed to make such information sharing acceptable."
He added: "The government appears to be using the London cameras as a Trojan Horse to secure unprecedented access to information on car drivers' movements without full public scrutiny or debate."
On Tuesday, the Home Office announced that anti-terror officers in London would be exempted from parts of the Data Protection Act.
The Metropolitan Police previously had to apply for access to congestion charge data on a case-by-case basis.
Home Secretary Jacqui Smith said the change was needed to deal with the "enduring vehicle-borne terrorist threat to London".
In a written ministerial statement, anti terror minister Tony McNulty said the scheme would be reviewed after three months to ensure personal privacy was being protected.
And he said the Metropolitan Police will have to produce an annual report for the government's data protection watchdog, The Information Commissioner.
But internal documents mistakenly circulated around Westminster by the Home Office contain details of a more wide-ranging plan to track journeys throughout England and Wales.
An annotated draft of Mr McNulty's statement revealed the development of the scheme over the past few months and the opposition mounted by the Department for Transport.
The electronic document revealed that the controversy level of the plan was rated "high" by civil servants.
One Home Office official wrote: "Civil rights groups and privacy campaigners may condemn this as further evidence of an encroaching 'big brother' approach to policing and security, particularly in the light of the recent e-petition on road-pricing.
"Conversely, there may be surprise that the data collected by the congestion charge cameras is not already used for national security purposes and may lead to criticism that the matter is yet to be resolved."
The document noted that the Department for Transport had "expressed concern about the potential for adverse publicity relating to... plans for local roads pricing".
At the time, the then Transport Secretary, Douglas Alexander - now International Development Secretary - was reeling from the impact of a 1.8m strong petition against road pricing on the Downing Street website.
A Home Office spokesman hit back at claims the documents reveal a disregard for public concern over civil liberties.
"The experience of the last few weeks has shown that this is a necessary tool to combat the threat of alleged vehicle-born terrorism.
"It is right that these decisions are not taken lightly. This submission shows that the decision was subject to consideration across government.
"We will develop proposals to be discussed across government to ensure that bulk Automatic Number Plate Recognition (ANPR) data-sharing with the police is subject to a robust regulatory framework which ensures public openness.
"No decision has yet been made on whether ANPR data from third parties should be made available in this way to the police for other crime fighting purposes. Such a decision would only be taken with wider consultation."