Google has blamed 'human error' for technical problems that left users believing they were unable to safely access search results at the weekend.
For more than half an hour on Saturday, every search result was accompanied by the message: "This site may harm your computer".
Google normally flags search results with the message if the site is known to install malicious software in the background.
However, "human error" in a Saturday update of the risky sites list caused all sites to be flagged, instead of only those sites identified by non-profit organisation StopBadware.org, according to a Google blog posting.
Marissa Mayer, vice president search products and user experience at Google apologised to search users for the inconvenience and to site owners whose pages were incorrectly labelled.
"We will carefully investigate this incident and put more robust file checks in place to prevent it from happening again," said Mayer.