Facial recognition tech has been used in shopping centres, museums and conference centres around the UK, according to a civil liberties group.
Big Brother Watch said the Meadowhall shopping centre in Sheffield had used the technology in a trial, and that it had also been used at Liverpool's World Museum and Birmingham's Millennium Point conference centre.
It comes after an investigation was launched this week into the use of the controversial technology at King's Cross in London.
"There is an epidemic of facial recognition in the UK," said Big Brother Watch boss Silkie Carlo.
She added: "We now know that many millions of innocent people will have had their faces scanned with this surveillance without knowing about it, whether by police or by private companies."
The company that owns the Meadowhall site, British Land, said in a statement: "We do not operate facial recognition at any of our assets.
"However, over a year ago we conducted a short trial at Meadowhall, in conjunction with the police, and all data was deleted immediately after the trial."
World Museum told Sky News it had used the technology during an exhibition on China's Terracotta Warriors in 2018 because there was "a heightened security risk".
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It said it was put in place after seeking advice from police and counter-terror advisers and was clearly communicated by signage around the venue.
"World Museum did not receive any complaints and it is no longer in use," said a statement from National Museums Liverpool.
Millennium Point's privacy police states it "sometimes" uses facial recognition "at the request of law enforcement authorities" – but the venue declined to comment further.
Data protection watchdog the Information Commissioner's Office said on Thursday that it was "deeply concerned" by facial recognition being used at the 67-acre King's Cross site.
London Mayor Sadiq Khan has also written to the development's boss to raise his concerns.
Argent, which operates the site, told the Financial Times it was one of "a number of detection and tracking methods" in place at King's Cross.
It also said there were "sophisticated systems" in place to protect privacy.
The House of Commons science and technology committee said last month that authorities should stop trials of facial recognition technology until a legal framework is set up.