Google has admitted it has been mistakenly collecting data from unsecured Wi-Fi networks for three years.
The company said the error came to light after the German data protection authority audited the Wi-Fi data collected by Street View cars for use in location-based products such as Google Maps for mobile.
The authority revealed that as well as collecting SSID information (the network's name) and MAC addresses (the number given to Wi-Fi devices such as a router), Google had also been collecting payload data such as emails or web page content being viewed.
"It's now clear that we have been mistakenly collecting samples of payload data from open Wi-Fi networks, even though we never used that data in any Google products," said Alan Eustace, senior vice president of Engineering & Research at Google, in a blog.
Google said the error came after a piece of code written in 2006 was included in the software used by its Street View cars.
The search also said typically it only collected "fragments of payload data" as the cars were always on the move and the "in-car Wi-Fi equipment automatically changes channels roughly five times a second".
As soon as Google became aware of the problem it grounded the cars and then separated the data and disconnected it from the network.
"We want to delete this data as soon as possible, and are currently reaching out to regulators in the relevant countries about how to quickly dispose of it," said Eustace. "Maintaining people's trust is crucial to everything we do, and in this case we fell short."
Google said it was now asking a third-party to review the software and confirm the data has been deleted.
The search engine also confirmed that it did not collect any data from secure password-protected Wi-Fi networks.
"The engineering team at Google works hard to earn your trust - and we are acutely aware that we failed badly here."