NEW YORK - BlackBerry maker Research in Motion Ltd. said an insufficiently tested software update at the company's network data centre was the cause of a service outage this week that left millions of users across North America without wireless email access.

In a statement late Thursday, the Canadian company said the outage from Tuesday evening into Wednesday morning was triggered by "the introduction of a new, non-critical system routine'' designed to optimize the cache, or temporary holding space, of the system that handles email sent to BlackBerry users.

RIM said it did not expect the update to impact users, "but the pre-testing of the system routine proved to be insufficient.''

The Waterloo, Ont.-based company also said the process designed to maintain service in the event of a failure "did not fully perform to RIM's expectations,'' causing a longer delay before service was restored.

RIM said it ruled out security and capacity issues, along with hardware failure or core software issues, as the cause of the disruption and is improving its testing, monitoring and recovery processes to prevent such an outage from happening again.

The problem occurred at RIM's hub for North American traffic, which routes messages to and from cellular service providers for the roughly 8 million BlackBerry devices now in use.

Many people rely heavily on the devices as a lifeline when they are away from their computers. Even more have simply become accustomed to -- and often obsessive about -- being able to check their email at night and on weekends.