"We know how many people rely on Gmail for personal and professional communications, and we take it very seriously when there's a problem with the service," Ben Treynor, Google's "Site Reliability Czar," said in a blog post.

"I'd like to apologize to all of you," Treynor said, adding that the 100-minute Gmail breakdown was "a Big Deal, and we're treating it as such."

Treynor said Google had taken a "small fraction" of Gmail's servers offline to perform routine upgrades but "slightly underestimated the load" that placed on other servers, which became overloaded.

"As a result, people couldn't access Gmail via the Web interface because their requests couldn't be routed to a Gmail server," said Treynor, a vice president of engineering.

"We've turned our full attention to helping ensure this kind of event doesn't happen again," he added.

"Gmail remains more than 99.9 percent available to all users, and we're committed to keeping events like today's notable for their rarity," he said.

Google's last major technical problem occurred in May, leaving millions of people unable to use popular services such as its main search page, as well as Gmail and Google News.

The hour-long outage affected Web surfers not only in the United States but in other countries and numerous other services including Google Reader, Google Maps, Google Analytics and video-sharing site YouTube.

Google also experienced a breakdown of Gmail, which has tens of millions of users, in February which lasted a few hours.

The failures, while rare, are seen by some industry analysts as a setback to the Mountain View, California-based Google's efforts to promote Gmail and other Web-based services to businesses.

(c) 2009 AFP