Despite previous denials, the search giant today confirmed that it did not in fact delete all the stolen data, which could include people's passwords, emails and even private web browsing histories.
The timing of Google's admission to a UK data regulator has been criticised as it comes when large parts of the global media is distracted with coverage of the start of the London 2012 Olympic Games.
Last month, the UK's Information Commissioner's Office (ICO) reopened its investigation into Google's admission that its Street View car harvested data from unsecured WiFi networks in UK homes in 2010, along with other countries.
The ICO previously dropped the probe after being told that only a limited amount of data had been "mistakenly collected" by the project.
Google had given assurances to the ICO over improvements to its privacy structure and policies, but the ICO decided to reopen its investigation after more information on the Street View breaches was released by the US Federal Communications Committee (FCC) in April.
The FCC found that the software to harvest payload data from unsecured home wireless networks was deliberately written by a Google engineer in 2006.
The engineer, not named but understood to be Marius Milner, had created special software that could "collect, store and review payload data for possible use in other Google projects".
Following today's admission by Google, a spokesman for the ICO said that the failure to delete the stored information is a "cause for concern", and ordered the data to be handed over.
Google has always claimed that the data collection was unintentional, despite the US FCC finding that senior managers on Street View were aware of the harvesting software.
Peter Fleisher, Google's global privacy counsel, wrote to the ICO today confirming that the firm "still has in its possession a small portion of payload data collected by [its] Street View vehicles in the UK".
He apologised for the error, and explained: "In recent months, Google has been reviewing its handling of Street View disks and undertaking a comprehensive manual review of our Street View disk inventory.
"That review involves the physical inspection and re-scanning of thousands of disks. In conducting that review, we have determined that we continue to have payload data from the UK and other countries. We are in the process of notifying the relevant authorities in those countries."
An ICO spokesperson said that Google's admission that information still exists "appears to breach the undertaking to the ICO signed by Google in November 2010", which said that all material should have been deleted by the following month.
Google said today that it wanted to delete the remaining data, but the ICO told the firm that it "must supply the data to the ICO immediately, so that [the ICO] can subject it to forensic analysis before deciding on the necessary course of action".
The spokesperson added: "We are also in touch with other data protection authorities in the EU and elsewhere through the Article 29 Working Party and the GPEN network to co-ordinate the response to this development.
"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 cause for concern."