Shoppers' faces are captured with a green box as they arrive at the store

But bosses at Asda, on Eastway, Fulwood, insist the facial detection software does not log personal data and is merely used to enhance image quality and deter thieves.

Those entering the store now see a bright green square around their heads on TV screens, leading to concern and anger from some customers.

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Shoppers' faces are captured with a green box as they arrive at the store

However, rather than tracking customers or scanning faces, Asda said the software works to focus the image – in much the same was as a digital camera or mobile phone would – and improve image quality.

No further data, beyond the CCTV image itself, is stored, the firm added. CCTV surveillance is commonplace across a range of stores.

The images are stored in the same way as regular CCTV and are collected as part of a trial at stores selected at random, Asda said.

Civil liberties campaign group Big Brother Watch said the cameras are “shameful” and called on the supermarket to scrap the scheme.

A screen in stores shows a live feed from the CCTV camera

It said: “Excessive surveillance systems like this are designed to intimidate shoppers and change their behaviour.

“This face detection system effectively puts shoppers in the crosshairs and is a shameful way of reminding customers they’re being watched. Customers ought to be treated with respect by retail giants like Asda, not like suspects.

“This overblown surveillance will likely deter many loyal customers. Asda should drop this offensive system as soon as possible.”

In a statement, Asda said: “Our cameras do not detect faces and the cameras are a deterrent to help prevent crime.

The CCTV debate - keeping us safe or an invasion of privacy?

“We do not use facial recognition and it would be false to suggest that.”

But it conceded the green boxes “help the camera focus on faces” as a “deterrent” to crooks.

Footage is kept for up to 60 days – or longer if a crime is caught on tape.

Walmart, which is based in the United States and owns Asda, last month confirmed it uses cameras at checkouts – that spot when items are put in bags without being scanned – in more than 1,000 stores.

In the UK, police forces in London and South Wales have been trialling the use of vans mounted with facial recognition cameras, parked at events and in highly-populated public places, that check the images of passers-by against a database of wanted people.

The move has attracted criticism and controversy, with a study by the University of Essex – where researchers were given access to six live trials by the Metropolitan Police – found matches were wrong in four out of five cases, and that the system was likely to breach human rights laws.

But the Home Secretary Sajid Javid said yesterday: “I back the police in looking at technology and trialling it and... different types of facial recognition technology is being trialled especially by the Met at the moment and I think it’s right they look at that.”

A spokesman for the Information Commissioner’s Office, the UK’s data watchdog, said: “Organisations that process personal data, including CCTV footage of identifiable individuals, need to ensure they comply with their obligations under data protection law.

“This includes letting people know that you are collecting the footage, keeping the images secure, and only disclosing the images when it is necessary to do so.”

A privacy statement on Asda’s website states: “We capture direct camera footage and images of you when you enter the field of vision of CCTV cameras in our stores, at Scan & Go and self-service checkouts, in our carparks, sites and Home Offices.