Research In Motion (RIM), the maker of the Blackberry has said an insufficiently tested software upgrade caused the widespread network failure that affected customers last week.

Blackberry user in North America were most badly affected last Tuesday and Wednesday when server outages made it impossible to retrieve emails.

The company said in a statement the failure was trigged by "the introduction of a new, non-critical system routine" designed to increase the system's email holding space.

It went on to admit that "the pre-testing of the system routine proved to be insufficient".

Blackberry has some eight million users worldwide and nearly half the worldwide market share of push email wireless devices according to analysts.