Canadian smartphone vendor RIM has reportedly cut internal sales estimates for its BlackBerry PlayBook by more than half for its second quarter, from 2.4 million units to 900,000, reports Digitimes.

The revision came after RIM announced it had sold 500,000 PlayBooks, which were sold from its mid-April launch until early June, but demand has continued to fall since its strong performance (50,000 units) on launch day.

It is hoped that as RIM releases 3G, LTE and WiMAX models of its Blackberry PlayBook in the third quarter, demand will increase and sales will begin to increase as the company can fully ready its attack on market leader Apple. However, Apple’s hold on the market is dominant, with sales expected to reach 10 million units in the second quarter, ten times that of its PlayBook rival.

Digitimes estimates that monthly shipments of the Motorola Xoom, Acer Inconia, Asus Transformer and PlayBook are averaging 100,000-200,000 units, suggesting each of Apple’s rivals will need to find new ways of increasing demand for their products.