Big Brother Watch has accused the UK's tax office of creating a database of 5.1 million people's voiceprints.Big Brother Watch says HMRC's Voice ID system has collected 5.1 million audio signatures and accuses the department of creating “biometric ID cards by the back door”.The Information Commissioner's Office (ICO) says it is following up complaints about the service.HMRC says the data is held securely.It also emphasised that callers could choose not to use Voice ID.The Voice ID scheme, which was launched last year, asks callers to repeat the phrase “my voice is my password” to register.HMRC says this process helps speed up security procedures and improves access to its digital services.But Big Brother Watch said taxpayers were being “railroaded into a mass ID scheme”, as they were not given the choice to opt out.”These voice IDs could allow ordinary citizens to be identified by government agencies across other areas of their private lives,” said Silkie Carlo, director of Big Brother Watch.The complaint comes two months after the EU's General Data Protection Regulation came into force. GDPR imposes greater restraint on organisations' use of personal data, particularly biometric data including voice samples. Big Brother Watch said that HMRC's voice ID had no lawful basis, because the collection of voice samples was “not necessary for its statutory function”.It also questioned whether HMRC had carried out a privacy impact assessment, as required by GDPR. The campaign group also claimed the collection would also have breached previous data legislation.