HMRC has been accused of building a biometric voice database of 5.1 million taxpayers without their explicit consent.
The database was built using what HMRC called its Voice ID service, in which callers to the organisation's self-assessment helpline were obliged to provide a Voice ID, without being given a clear choice to opt-out.
That's according to privacy group Big Brother Watch, which made the claims in an investigation.
While it was possible to avoid opting in to Voice ID by shouting ‘no' three times in response to the automated helpline demand, HMRC didn't provide any apparent way for taxpayers to opt out, while the nature of the phone line meant that most callers could simply put the phone down either.
"Voice ID technology is a form of biometric identification and authentication, as sensitive as a fingerprint," explained Big Brother Watch.
Its report continued: "Voice recognition technology is used to extract and analyse unique voice patterns and rhythms to identify a person using just their voice, checking over 100 behavioural and physical vocal traits, including the size and shape of your mouth, how fast you talk and how you emphasise words."
Big Brother Watch used a series of Freedom of Information requests to find out how many biometrics HMRC now holds, noting that the initiative essentially created "biometric ID cards by the back door".
Furthermore, noted the organisation, HMRC has refused to disclose which other government departments the Voice ID database might have been shared with, how they are stored and used, or if it's possible to delete a voice ID once an account has been established.
Silkie Carlo, director of Big Brother Watch, said: "Taxpayers are being railroaded into a mass ID scheme that is incredibly disturbing. The tax man is building Big Brother Britain by imposing biometric ID cards on the public by the back door.
"The rapid growth of the British database state is alarming. These voice IDs could allow ordinary citizens to be identified by government agencies across other areas of their private lives.
"HMRC should delete the five million voiceprints they've taken in this shady scheme, observe the law and show greater respect to the public."
Big Brother Watch added that it flagged the issue with the Information Commissioner's Office, which is now investigating HRMC's Voice ID scheme.
HMRC, meanwhile, claimed that Voice ID had proved popular with taxpayers and that it "meets the highest government and industry standards for security", which doesn't exactly answer the many questions that still haven't been answered about the biometric project.
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