After being caught spying on people across Europe and Australia with its Wi-Fi-slurping Street View cars, Google had told angry regulators that it would delete the ill-gotten data.
Google broke its promise.
Britain's Information Commissioner's Office (ICO) received a letter from Google in which the company admits it kept a "small portion" of the electronic information it had been meant to get rid of.
"Google apologizes for this error," Peter Fleischer, Google's global privacy counsel, said in the letter, which the ICO published on its website.
The ICO said in a statement that Google Inc. had agreed to delete all that data nearly two years ago, adding that its failure to do so "is cause for concern."
Other regulators were less diplomatic, with Ireland's deputy commissioner for data protection, Gary Davis, calling Google's failure "clearly unacceptable." Davis said his organization had conveyed its "deep unhappiness" to Google and wants answers by Wednesday.
Google said that other countries affected included France, Belgium, the Netherlands, Norway, Sweden, Finland, Switzerland, Austria and Australia. Attempts to reach regulators in several of those countries weren't immediately successful Friday.
Google angered officials on both sides of the Atlantic in 2010 when it acknowledged that its mapping cars, which carried cameras across the globe to create three-dimensional maps of the world's streets, had also scooped up passwords and other data being transmitted over unsecured wireless networks. Investigators have since revealed that the intercepted data included private information including legal, medical and pornographic material.
The Mountain View, California-based company had been meant to purge the data, and Google chalked up its mistake to human error.
The company said it recently discovered the data while undertaking a comprehensive manual review of Street View disks. The company said it had contacted regulators in all of the countries where it had promised to delete data but realized it had not.
Fleischer's letter asks Britain's ICO for instructions on how to proceed; the ICO told Google that it must turn over the data immediately so it can undergo forensic analysis.
Friday's disclosure comes just over a month after the ICO reopened its investigation into Google's Street View, saying that an inquiry by authorities in the United States raised new doubts about the disputed program.
In April, the U.S. Federal Communications Commission fined Google, saying the company "deliberately impeded and delayed" its investigation into Street View.
It's unclear what, if any, penalties would be imposed on Google by Britain's ICO or regulators in any of the 10 other jurisdictions in which the company had wrongly retained Street View data.
"We need to take a look at the data... There's all sorts of questions we need to ask," an ICO spokesman said, speaking on condition of anonymity because office rules prohibit him from being named in print.
The ICO has the power to impose fines of up to 500,000 pounds (roughly $780,000) for the most serious data breaches, although penalties are generally far less severe and can involve injunctions or reprimands.
Explore further: Tracking your digital fingerprint online raises privacy issues
Google's letter to Britain's watchdog: bit.ly/OrXSd3
British watchdog's response to Google: bit.ly/OrXYRV