Google has admitted that it had not deleted users' personal data gathered during surveys for its Street View service.
The data should have been wiped almost 18 months ago as part of a deal signed by the firm in November 2010.
Google has been told to give the data to the UK's Information Commissioner (ICO) for forensic analysis.
The ICO said it was co-ordinating its response with other European privacy bodies.
In May 2010 it was revealed that Google had scooped up about 600 gigabytes of personal data from unsecured wireless networks while gathering images and location data for Street View.
The data was collected for years in 30 countries while Google compiled information for the mapping service.