More than 160,000 people have instructed HMRC to delete biometric data it captured during phone calls.
The tax authority gathered millions of callers' voice data after launching its Voice ID system in 2017. However, callers were originally given no obvious way to opt out. Privacy campaign group Big Brother Watch said people had been "railroaded into a mass ID scheme by the back door".
HMRC now allows callers to opt out or decline Voice ID – which allows taxpayers to use their voice instead of a password when calling the helpline – and delete captured "voiceprints". Despite this, around seven million Voice ID records have been kept in a database held by a third party.
A Freedom of Information request revealed that 162,185 people had opted out and had their voice data deleted by December 2018.
Prior to the current system, callers needed to get in touch with HMRC separately and ask to be removed from the Voice ID system, which around 80 people did.
Big Brother Watch has reported HMRC to the Information Commissioner's Office (ICO), claiming it had broken data protection law.
A statement from the group said: "It is down to the ICO to take robust action and show that the government isn't above the law."
An HMRC spokesman said: "Our Voice ID system is very popular with millions of customers as it gives a quick route to access accounts by phone.
"All our data is stored securely and customers can opt out of Voice ID or delete their records any time they want."
The organisation is cooperating with the ICO, which is investigating to see whether the system breached GDPR.
