Information Commissioner Christopher Graham confirmed Google was in "significant breach" of the Data Protection Act, but said the search engine would not face a fine if it deleted all of the data and ensured it never happens again.
"It is my view that the collection of this information was not fair or lawful and constitutes a significant breach of the first principle of the Data Protection Act," said the Information Commissioner.
"The most appropriate and proportionate regulatory action in these circumstances is to get written legal assurance from Google that this will not happen again — and to follow this up with an ICO audit."
In May this year an audit of wi-fi data collected by the search engine's Street View Cars, which capture real-time photographs of cities across the world for use in Google Maps, by the German data protection authority revealed Google has been accidentally collecting snippets of data.
The ICO then visited Google to "make a preliminary assessment of the 'pay-load' data it inadvertently collected".
"Whilst the information we saw at the time did not include meaningful personal details that could be linked to an identifiable person, we have continued to liaise with, and await the findings of, the investigations carried out by our international counterparts," the ICO said.
However, last week Google admitted that the inadvertent wi-fi snooping collected not only data fragments but entire email messages, website addresses and passwords.
In light of the new findings the ICO said it has already made enquires to see whether they relate to the data inadvertently captured in the UK.
"We are in the process of confirming that there are no outstanding legal obligations upon us to retain the data, and will then ensure that it is quickly and safely deleted," Peter Fleischer, global privacy counsel for Google, told The Guardian.
"Since we announced our mistake in May we have cooperated closely with the ICO and worked to improve our internal controls. As we have said before, we did not want this data, have never used any of it in our products or services, and have sought to delete it as quickly as possible," Fleischer said.
In New Zealand, the Street View programme has been cleared of privacy concerns, following a police investigation.