If you drive in London, chances are a camera will see you.

The city uses a network of Automated Number-Plate Recognition (ANPR) cameras to scan the roads for vehicles entering and leaving its road-charging zones.

The UK road system is carpeted with these cameras.

Every day, around 13,000 capture 55 million “reads”, as number plate identifications are known, according to figures from the National Police Chiefs’ Council.

But nowhere are they more densely packed than the capital city, which is believed to have around 2,000 cameras, sending tens of millions of reads a day back to their operator, Transport for London.

Now the mayor of London has given police access to more data from a larger number of cameras, and privacy campaigners are up in arms.

“It’s kind of terrifying,” says London Assembly member Sian Berry, who along with privacy campaigners the Open Rights Group is bringing a legal challenge against the mayor’s decision.

They warn that although scans of car number plates may seem innocent, they are not.

Firstly, because a record of a vehicle’s journey is an intimate insight into a driver or passenger’s movements.

Secondly, because ANPR cameras do not just scan for numbers and letters, they also take pictures, including a “front of vehicle photo” taking in everything that happens to be around when the image is snapped.

This includes the colour and make of vehicles, and potentially the faces of drivers and passing pedestrians – what is known by the London authorities as “enhanced contextual data”.

Previously, the Metropolitan Police only had access to data from ANPR cameras in central London and did not receive any images, only “reads” of where and when a number plate was picked up.

Now the force has been given full access to cameras across inner London, an area where far more people live than central London (3.8 million as compared to 200,000), and it will be able to see photographs as well.

City Hall did not respond to a request for comment, but the Metropolitan Police defended the need for the data, saying it helped the police protect the public and avoid errors in identifying vehicles.

ANPR images were “extremely unlikely to be of sufficient quality to identify the driver or passengers”, the force said, adding that in any case Londoners can have “little expectation of privacy” when driving their cars.

Ms Berry is more specific. She says the additional access creates the prospect of a privacy campaigner’s worst nightmare: a database filled with deeply personal data which can be searched by police whenever they want.

“We do know that there have been police disciplined and expelled for stalking their ex-partners using data that the police hold,” she says.

“When there aren’t proper internal controls, it really increases the risk of that kind of harm.”

Ms Berry points out that the police can get data from ANPR cameras for an investigation, a power that was used 33,000 times by the Metropolitan Police in 2020 alone, but they have to request and give reasons for using the data.

The access given by the mayor could create a database for police to “play with”, she says, noting that it would be simple to run facial recognition scans on the pictures.

In a letter to the mayor notifying him that they intend to take legal action, Ms Berry and Open Rights Group argue that the decision to extend the Metropolitan Police’s powers in this way was illegal, because it was granted without proper consultation.

When Sadiq Khan authorised the access in May this year, he cited a public consultation held in 2014, an exercise that the campaigners and their lawyers at Bindmans argue cannot account for such a large-scale increase in police access.

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“With a stroke of a pen, Sadiq Khan has taken a decision that violates the basic privacy rights of millions of Londoners,” says Jim Killock, executive director of the Open Rights Group, which is calling on the mayor to hold a full-scale public consultation on the move.

Mr Killock fears that there could be worse to come, as the mayor plans to expand the Ultra Low Emission Zone to cover the whole of Greater London from the end of 2023, significantly increasing the number and scope of ANPR cameras.

If this happens, he says, “every single car, driver and pedestrian in Greater London will be subject to surveillance by the Metropolitan Police, yet Londoners have had no say in this”.