The UK Information Commissioner's Office (ICO) has given Google a rap on the knuckles for the collection of payload data, including email addresses, URLs and passwords, during the collection of information by its Street View cars.
It has served an enforcement notice for the offence, and Stephen Eckersley, ICO Head of Enforcement, said Google has 35 days to delete the data or find itself in the dock for contempt of court.
The company collected the data from Wi-Fi networks as its cars drove around the country between 2007-10. In 2012 it admitted that it had accidentally retained four disks' worth of data, but it was quarantined rather than deleted.
The decision was made after the ICO found that Google collected the data through a "procedural failing" and "serious lack of management oversight".
It has not leveled a fine on Google, but said it will be watching it closely in the future.
Eckersley said: "The early days of Google Street View should be seen as an example of what can go wrong if technology companies fail to understand how their products are using personal information. The punishment for this breach would have been far worse, if this payload data had not been contained."
The ICO is still looking into whether Google has violated the Data Protection Act.