(AP) -- Google Inc.'s Gmail service was knocked offline Tuesday in a severe outage that the company said affected a "majority" of users.

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

representatives did not immediately return messages left by The Associated Press.

Mountain View-based Google notified users in a posting on the company's Web site that problems with Gmail surfaced Monday, wiping out to a "small subset" of users. On Tuesday afternoon the company said an outage affecting "a majority of users" had hit . Google didn't say whether the outages were related.

Google restored service to some users. The company said it expected to have the problem fixed later Tuesday.

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.

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