Google could face heavy fines under Britain's privacy laws after it admitted copying computer passwords and entire emails from households across the UK.
The US firm says the personal data was picked up from wireless networks when its fleet of cars drove down residential roads taking photographs for its highly controversial Street View project.
Millions of homes could be affected, and anti-surveillance campaigners have described the error as "outrageous".
Britain's privacy watchdog, the Information Commissioner's Office, has launched an investigation, and is considering taking action against the search engine company.
Six months ago, the ICO was granted new powers by Gordon Brown's government, including the ability to impose fines of up to £500,000 for breaches of privacy.
However, this is unlikely to ruffle a firm with a market capitalisation of nearly $200bn (£127bn).
A spokesman for the ICO said: "Earlier this year the ICO visited Google's premises to make a preliminary assessment of the 'pay-load' data it inadvertently collected whilst developing Google Streetview.
"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.‬
"Now that these findings are starting to emerge, we understand that Google has accepted that in some instances entire URLs and emails have been captured.
"We will be making enquiries to see whether this information relates to the data inadvertently captured in the UK, before deciding on the necessary course of action, including a consideration of the need to use our enforcement powers."‬
Alan Eustace, Google's vice-president of engineering and research, said: "We want to delete this data as soon as possible, and I would like to apologise again for the fact that we collected it in the first place.
"We are mortified by what happened."
Alex Deane, director of privacy campaign group Big Brother Watch, said the Google Street View project, launched in 2008, should never have been carried out in the first place.
He added: "As if building up a database of photographs of millions of people's private homes wasn't enough, the news that Google has also harvested email addresses and passwords is nothing short of outrageous.
"Google must launch an urgent investigation as to how this gross invasion of privacy was allowed to happen."
Privacy watchdogs in numerous countries, including France, Germany and Canada, have also investigated the blunder.
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