Google Inc.'s Gmail service was knocked offline Tuesday in an outage that the company said affected a "majority" of its millions of email users, including consumers who get Gmail for free and businesses that pay for a version for their employees.

The disruption, which lasted under two hours, was a reminder of the growing dependence on Google's technology. The free version of Gmail is the world's third most-popular email program with some 149 million users worldwide in June, ranking behind the free email services offered by Microsoft Corp. and Yahoo Inc., according to data from comScore Inc.

Google only says Gmail is used by tens of millions of people.

Though occasional disruptions are common, widespread outages involving Google's services are rare. They are becoming a bigger threat to Google as it tries to sell more of its services to businesses.

Businesses are increasingly leaning on Google's services because they are delivered over the internet instead of being managed in-house. That can save companies money and buy them more storage than they could otherwise afford. But many corporations are skeptical about outsourcing such critical tasks.

Google argues that web-based services are more reliable than those handled in-house, but big outages such as Tuesday's add another challenge to selling to reluctant businesses.

Google says more than 1.75 million businesses use Gmail as part of Google Apps, which is Google's answer to business software from Microsoft Corp. It's a key part of Google's strategy to inject its brand deeper into corporations. Google Apps costs $50 US per user per year.

It wasn't immediately clear what caused Tuesday's disruption, which led Gmail users to get an "Unable to reach Gmail" error message as their computers tried repeatedly to reconnect to the service. Google said it was investigating.

As usually happens with hiccups in popular web-based services, the Gmail glitch touched off a flurry of posts to social-networking websites from frustrated Gmail users wondering whether others were having the same problem.

"Omg thank God I was at work!!! I would've gone mad!" one Twitter user wrote. Some favoured snark. "Minute 30 of Gmail outage. The cities are in flames and people eating pets to survive. To future generations: we meant well," another user tweeted.

The last major outage at Google happened in May, when millions of people were cut off from Google's search engine, email and other online services after too much traffic was routed through computers in Asia. About 14 per cent of Google's users encountered problems with the internet's No. 1 search engine. Those outages lasted about an hour.

Google had two separate outages this week. One was a small outage on Monday that wiped out email to only a "small subset" of users. Tuesday afternoon's outage affected "a majority" of Gmail users, the company said, and lasted about an hour and 45 minutes.