Google is having problems keeping its uptime pledge to some paying customers of its Google Apps suite of hosted services, throwing into question the company's ability to offer guaranteed levels of application reliability.

Little over a month after introducing Google Apps' Premier version, which includes a 99.99 percent uptime commitment, Google is failing to meet that service level agreement (SLA) for an undetermined number of customers.

"Google has not met its SLA with me, that's correct," Grant Cummings, a US-based IT professional and Premier customer said.

On Tuesday, Google Apps' Gmail service suffered significant availability problems that began in the morning (US Eastern Time) and were declared officially solved for all users early Wednesday afternoon. The problems also affected Gmail users who aren't on Google Apps.

Cummings has experienced hour-long Gmail outages twice in recent weeks. And to add to the frustration, the higher support level his Premier status affords him, such as phone assistance, has been underwhelming. "I have called Google support and, while they've been very cheerful, easily accessible and friendly, they have been pretty much useless," he said. "A little more information for the paying customers as opposed to not deviating from the generic canned responses would be appreciated."

Brian J.S. Miller, a US-based web design and hosting service provider is on the Standard version, but expects that when he upgrades to Premier, Google will be able to honour its uptime commitment. "Yes, I was affected [by Tuesday's Gmail downtime]. It was a big problem. I was very disappointed," Miller, who is otherwise happy with Google Apps.

Google's introduction in February of the Premier edition, which costs $50 (£25.47) per user per year, generated much attention, as many saw it as yet another concrete step in the company's march toward a direct confrontation with Microsoft in the market for productivity and collaboration applications where Microsoft Office reigns. Google's hosted, software-as-a-service approach contrasts with Microsoft's traditional packaged software model.

To make up for the first major Gmail incident, which occurred 1 March, Google extended Premier customers' contracts at no extra charge. Another Gmail problem hit on March 12.

Early Wednesday afternoon, a Google spokesman said that "a number of Gmail users have had difficulty accessing their accounts and sending mail over the past day. We have now resolved the problem, and we apologise for the inconvenience this may have caused."

Google is currently offering customers the opportunity to try the Premier edition for free until the end of April.