Google vowed to step up monitoring of its employees after firing an engineer who rifled through children's email accounts and phone records.
David Barksdale, a 27-year-old "site reliability engineer" with access to Google customers' private accounts, was fired after parents and children complained that he had used to the data to harass four minors, including a 15-year-old boy he had befriended.
The incident was the second time a Google employee had been fired for such a breach of trust, the company confirmed, and said it was acting quickly to prevent the incidents denting public confidence in its security.
Mr Barksdale met the teenagers through a computer club in Seattle where he worked, and boasted of having access to most of the personal communications of Google users.
He was able to read messages to and from customers' Gmail accounts and conversations on the Gtalk instant messaging service, as well as phone records of users of the new Google Voice internet phone service.
In one incident, Mr Barksdale was accused of accessing a 15-year-old boy's accounts to find out the name and telephone number of his girlfriend, and then taunting him with threats to call her. In another, he is alleged to have unblocked himself from one person's Gtalk contacts list, after that person tried to stop him getting in contact.
"We dismissed David Barksdale for breaking Google's strict internal privacy policies," said Bill Coughran, the company's senior vice-president for engineering. "We carefully control the number of employees who have access to our systems, and we regularly upgrade our security controls. A limited number of people will always need to access these systems if we are to operate them properly, which is why we take any breach so seriously."
A Google spokesman later refused to reveal how many of the company's 20,000 employees have access to clients' personal accounts. No details were available about the earlier spying incident.
In an email exchange with the website Gawker, which first reported the privacy breach, Mr Barksdale wrote: "You must have heard some pretty wild things if you think me getting fired is newsworthy."
Revelations of privacy breaches come as Google is trying to persuade internet users to share even more of their personal information with the company and to use its services for social networking and for playing games with friends. Eric Schmidt, its chief executive, this week called on Facebook to allow its users to share their list of friends with Google, which is secretly working on a major social networking project, believed to be codenamed Google Me.
In common with other internet sites, to whom users increasingly trust large amounts of personal information and communications, Google has stepped up its attention to privacy concerns. In June, it published a white paper setting out how it works to protect customer data, including how it stores data in fragments across different servers to prevent outside hackers from piecing it together.