Struggling Canadian smartphone maker BlackBerry has fallen deeper into the red in the third quarter, reporting a massive $US4.4 billion ($A4.98 billion) loss.

Sales are plummeting, with third quarter revenues of just $US1.2 billion, 56 per cent lower than a year earlier.

BlackBerry attributed the extent of the quarterly loss to a $US4.6 billion charge for an inventory write-down and other one-time costs.