Google had not kept its promise to delete the personal data collected from its Street View cars in Britain and other countries in 2010, it said last week.
The US company admitted in May 2010 that its vehicles — which photograph neighbourhoods to create street-level images — had accidentally collected data from unsecured wireless networks used by residents in more than 30 countries.
Google said it had found remaining data when it manually searched its Street View disk inventory.
"Google has recently confirmed that it still has in its possession a small portion of ... data collected by our Street View vehicles in the UK," Peter Fleischer, Google's global privacy counsel said in a letter published by Britain's Information Commissioner's Office (ICO).
"Google apologises for this error."
Google said it was in the process of notifying relevant authorities in other countries.
The ICO said Google's ongoing retention of the data appeared to breach an undertaking it signed in November 2010.
"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 a cause for concern," it said.
The ICO told Google it must supply the data immediately so it could be subjected to forensic analysis before the office decided on the necessary course of action.
The search giant was fined $25,000 in April for impeding a US investigation into Street View data collection.