Gmail was unavailable for more than 90 minutes yesterday, an outage that may have affected almost 4.8 million users of the Google webmail service.

Google first acknowledged the problem around 4pm, and declared it resolved at 5.50pm, saying it had affected "less than" 1.38% of the Google Mail user base.

If the outage hit 1.37% of Gmail's approximately 350 million active users, the total number of affected users would have been about 4.79 million.

Users impacted were unable to access their Gmail accounts, according to the Google Apps Status dashboard.

Earlier this week, an undetermined number of Google Spreadsheets users were affected for about two hours by a bug that displayed frequent CAPTCHA screens when they attempted to open files. CAPTCHA systems ask users to enter text they display in a box before giving them access to specific functionality in an application. The purpose is to make sure that the users are human beings and not machines that may attempt to misuse or abuse the application in question.

Gmail suffered an outage in mid-April that lasted more than one hour and may have impacted more than 33 million users.

Gmail is used by individuals and, as a component of the Google Apps email and collaboration suite, by organizations. Gmail is free as a standalone service and as part of the Standard and Education editions of Google Apps. The Google Apps for Business edition costs $50 per user, per year, or $5 per user, per month for organisations that would rather renew their contracts monthly. More than 4 million businesses use Google Apps.

Reliability of cloud-based software is an ongoing concern for organisations, especially when a critical workplace tool like email is affected. Still, Google and other cloud software providers continue to steadily improve the performance and uptime of their products.