Google has blamed server maintenance for a widespread Gmail outage that delayed access to the web-based email service for more than an hour Tuesday.

Writing on the company blog, Google's site reliability "czar" Ben Treynor apologized for the disruption and said the company would take steps to ensure the outage doesn't happen again.

"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," he wrote.

The outage, which lasted 100 minutes, affected a majority of Gmail's millions of users.

The free version of Gmail is the world's third-most-popular email program with about 149 million users worldwide, according to to data from comScore Inc. It trails similar free services offered by Microsoft Corp. and Yahoo Inc.

Traynor said the problem began when the company took a small number of servers offline to perform routine maintenance, but underestimated the burden new changes to Gmail would have on request routers that direct traffic to Gmail servers. The request routers became overloaded and stopped accepting requests, putting more burden on other request routers and starting a cascade that overloaded all of them in minutes.

"As a result, people couldn't access Gmail via the web interface because their requests couldn't be routed to a Gmail server," he wrote.

Widepsread outages of Google's services are rare, but the threat of them could undermine Google's efforts to furnish more of its services to businesses.

Google says more than 1.75 million businesses use Gmail as part of Google Apps, business software the company sells in competition with Microsoft's Office software.