The US Federal Trade Commission fined Google $22.5 million for violating the privacy of people who used rival Apple's Safari web browser even after pledging not to do so.
The FTC said Google had agreed with the commission in October 2011 not to place tracking cookies on or deliver targeted ads to Safari users, but then went ahead and did so.
"For several months in 2011 and 2012, Google placed a certain advertising tracking cookie on the computers of Safari users who visited sites within Google's DoubleClick advertising network," the FTC said in a statement.
"Google had previously told these users they would automatically be opted out of such tracking."
"No matter how big or small, all companies must abide by FTC orders against them and keep their privacy promises to consumers, or they will end up paying many times what it would have cost to comply in the first place," said Jon Leibowitz, FTC chairman.
David Vladeck, director of the FTC's Bureau of Consumer Protection, questioned Google's efforts on privacy protection. "It is troubling to us that Google says, we didn't know," he told reporters.
"The answer on Street View was, we didn't realise what was going on. Their answer here is, we didn't know."
"A company like Google that is storing personal information from hundreds of millions of people has to do better."
"As regulators it is hard to know which answer is worse, I didn't know or I did it deliberately," he added.
While Google agreed to the fine, it did not admit it had violated the earlier agreement.
A Google spokesperson said the FTC was focused on a help center web page published more than two years before Google agreed to refrain from the cookie activities on Safari.
"We have now changed that page and taken steps to remove the ad cookies, which collected no personal information, from Apple's browsers," the spokesperson said. "We set the highest standards of privacy and security for our users."