Google disclosed Friday in an email to the U.K. Information Commissioner's Office that it had not yet deleted all user data collected by its Street View vehicles, as it had agreed to more than 18 months ago.
The Street View vehicles sniffed the content of users' Internet communications on open Wi-Fi networks.
Google said it discovered "a small portion of payload data ... from the UK and other countries." The email, which the ICO has made public, came from Peter Fleischer, Google's global privacy counsel.
The data was discovered during a manual review of thousands of disks, according to the email. A person familiar with the matter said human error was to blame for its continued existence.
The ICO said the data was supposed to have been deleted in December 2010 according to a November agreement between Google and the ICO. It was collected prior to May 2010, according to the agency.
The ICO re-opened its investigation into Street View earlier this year, after the U.S. Federal Communications Commission revealed in a report that Google knew its vehicles had collected user data. The FCC fined Google US$25,000 for impeding its investigation.
"The ICO is clear that this information should never have been collected in the first place and the company's failure to secure its deletion as promised is cause for concern," the ICO said in a statement Friday on its website.
A company spokesperson declined to comment further.
Google asked the ICO for permission to delete the remaining payload data in its email. However, the ICO notified Google that the body intends "to examine the contents of the remaining payload data."
Fleischer indicated in his email that Google would be willing to allow the ICO to inspect the data.
The company has also faced inquiries over data collection in other countries, including Spain and Italy.
Cameron Scott covers search, web services and privacy for The IDG News Service. Follow Cameron on Twitter at CScott_IDG.