Google's Gmail service was unavailable starting around 10:30 AM GMT on Tuesday, but it appeared to come back online for most users around two hours later.
The scope of the outage was not immediately clear, but at least some users in Europe and Asia on Tuesday could not get access to their inboxes or had to wait a minute or more for them to open.
"We're aware of a problem with Gmail affecting a number of users," Google said in an advisory on its Gmail support site. "We're working hard to resolve this problem and will post updates as we have them. We apologize for any inconvenience that this has caused."
As of filing time the message had not been updated.
Google has had trouble with Gmail before, setting off waves of concerns over the reliability of the service.
Last August, Gmail had three significant outages that affected not only individual consumers of the free Web mail service but also companies and organizations paying for Apps Premier, the company's hosted suite of collaboration, messaging and office productivity services. Apps Premier costs US$50 per user per year.
To compensate for the downtime, Google decided to extend a credit to all Apps Premier customers and also said it would get better at notifying people of problems. Google offers a 99.9 percent uptime guarantee for Gmail for users paying for Google Apps Premier.
An outage on Aug. 11 lasted about two hours and affected almost all Apps Premier users. The other two, on Aug. 6 and Aug. 15, hit a small number of Apps Premier users but locked some users out of their accounts for more than 24 hours.