Business and consumer users of Google's popular e-mail service were hit by a blackout on Tuesday.

The service went offline at 0930 GMT and began to return for many users after four hours, one of the longest downtimes ever suffered by Google.

More than 113 million people use Google mail worldwide, according to comScore.

Google said it apologised for the inconvenience the e-mail blackout had caused its users.

"We know how important GMail is to our users so we take this very seriously," it added.

According to comScore, Google has the world's third most popular web mail service behind Hotmail with 283 million users and Yahoo with 274 million e-mail users.

Professional suite

More than a million business around the world use Google's professional suite of applications, including e-mail. Google itself relies on the service and press spokespeople for the firm were unable to e-mail journalists with statements regarding the problem.

Professional users are covered by a service level agreement that promises to be 99.9% operational in any calendar month.

The "premier edition" of the Apps service costs $50 (£34) per user for a year.

According to Google, its e-mail service suffered an average of 10 to 15 minutes of downtime per month in 2008.

The last outage of note was in August 2008 when users were unable to use Google Mail for "a couple of hours".

After the incident Todd Jackson, product manager for Google Mail, said in a blog post: "We're conducting a full review of what went wrong and moving quickly to update our internal systems and procedures accordingly."

On its support page for Google Mail, the firm said "A number of users have had difficulty accessing GMail today.

"The majority are now able to access their e-mail accounts again and we're hoping to have service restored for the remainder very soon."