Google has admitted to installing hidden microphones in a home product and not telling customers they were there.
The Nest Secure alarm system did not include the microphone as part of its specifications but it was "never intended to be a secret", Google said.
The hardware feature came to light after Google announced a software update to the Nest Secure system would enable it to use its voice-activated Google Assistant, which is powered by artificial intelligence to answer queries and commands.
Until then, the product web page for the device did not say it contained a microphone. It has since been updated.
A Google spokesman said: "The on-device microphone was never intended to be a secret and should have been listed in the tech specs. That was an error on our part. The microphone has never been on and is only activated when users specifically enable the option.
"It is hard to believe Google cares about people's privacy after selling a security product with a secret microphone in it," said Silkie Carlo, director of UK-based privacy campaign group Big Brother Watch.
“售卖一款藏着隐形麦克风的安全产品的行为，实在叫人难以相信谷歌当真关心大众隐私。”英国民间维权机构Big Brother Watch负责人斯尔吉卡洛这样说。
"This appears to be deceptive rather than a 'mistake', which is incredibly damaging for public trust in Google. Many of our worries about smart home devices appear to be proving true.“