The government is considering plans to allow police permanent access to data from traffic cameras to track the movement of cars, according to a Home Office document released in error.

The “Big Brother” plans emerged after Home Office Minister Tony McNulty announced that real-time data from London’s 1,500 congestion charge cameras would be given to Scotland Yard under a temporary lifting of data protection rules.

Under current arrangements, the Metropolitan Police have to ask for data from the number-plate reading cameras on a case-by-case basis.

Background notes inadvertently attached to McNulty’s statement on Tuesday revealed Home Office plans to make the exemption from data protection plans permanent and to extend it across the country.

The Liberal Democrats called for a full debate on the issue, saying the London plans were a “Trojan Horse” to allow police to monitor drivers’ movements without public scrutiny.

McNulty said the “bulk transfer” of London’s car-tracking data was needed to fight the threat to the capital from car bombs.

At the end of June, two fuel-packed cars were discovered in London’s West End but the document reveals the proposals to give police the camera data had been under discussion months earlier.

It also shows the plans were resisted by transport ministers who feared it could add to public concerns over proposals for a national scheme for road pricing.

Earlier this year, 1.8 million people signed a Downing Street online petition calling for the road pricing scheme to be scrapped.

The document reveals that then Transport Secretary Douglas Alexander was concerned the plans “might be seen as colouring the debate about road-charging (that material being collected for traffic purposes is being used for other outcomes)”.

It also noted that civil rights groups and privacy campaigners “may condemn this as further evidence of an encroaching ‘big brother’ approach to policing and security”.

“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 added.

A Home Office spokesman said no decision had been taken whether automatic car recognition data should be given to police and that a decision would only follow wider consultation.

“The experience of the last few weeks has shown that this is a necessary tool to combat the threat of alleged vehicle-borne terrorism,” he added.