Google has confessed it has failed to stick to an agreement to delete all personal Wi-Fi data they 'mistakenly' collected via their Street View cars in 2010.
In a letter to the Information Commissioner's Office (ICO) Google has apologised for the error and sought advice how to proceed.
A row broke out in 2010 after it was revealed Google's cars had collected 600 gigabytes of private data from unsecured wireless networks around the world.
In a written agreement with the ICO in 2010, Google promised to delete all the information by the end of the year. But now the search engine has discovered not all the data was erased.
An ICO spokesman said: "Earlier today Google contacted the ICO to confirm that it still had in its possession some of the payload data collected by its Street View vehicles prior to May 2010.
"This data was supposed to have been deleted in December 2010.The fact that some of this information still exists appears to breach the undertaking they gave to the ICO in November 2010.
"In its letter to the ICO today, Google indicated that it wanted to delete the remaining data and asked for the ICO's instructions on how to proceed.
"Our response, which has already been issued, makes clear that Google must supply the data to the ICO immediately, so that we can subject it to forensic analysis before deciding on the necessary course of action.
"The ICO has always been clear that this should never have happened in the first place and the company's failure to secure its deletion as promised is cause for concern."
Google infuriated privacy campaigners in 2010 after they admitted their Street View cars, which carry panoramic cameras to collect pictures for their mapping service, had also scooped-up private web traffic, including passwords.
Peter Fleisher, Google's privacy counsel, said: "Google has recently confirmed that it still has in its possession a small portion of payload data collected by our Street View vehicles in the UK.
"Google apologises for this error."