An analyst has predicted that Apple will not reach forecasted Mac sales in the fourth fiscal quarter of 2012, due to the increase in people opting to buy an iPad instead.

In a note to investors on Tuesday, BMO analyst Keith Bachman said that the last two quarters of 2012 will be “challenging” for Apple, largely due to the expectation of a new iPhone launch, which leads to less iPhone 4S sales while customers wait for the new model to arrive.

Bachman’s iPhone sales estimate for the next six quarters has risen by 1.5 million units, though, as has his iPad estimate, which has increased by 3.8 million units. On the other hand, his Mac prediction has decreased.

“Net, we are lowering our Mac forecast by 1.6 million units over the next six quarters. We think strong iPad demand is negatively affecting Mac sales, despite the recent MacBook refresh,” wrote Bachman. “For the next six quarters, we reduced our Mac sales by 40 per cent of the increase in iPad sales.

Bachman’s fiscal 2012 Mac estimate is now 18.22 million units, and his 2013 estimate is 20.18 million units, down from his previous prediction of 21.38 million.

Bachman does ensure investors that they shouldn’t be concerned about these predictions. “We think investors are well aware of the product cycle, and likely negative revision to near-term estimates,” he wrote. “Moreover, we think Apple shares will respond in a positive way to the pending launch of the new iPhone, as we look to year-end.”

“If Apple is able to launch the new iPhone in the month of September, then actual unit shipments in the September quarter will be less relevant, as management and investors will focus on the momentum of the new iPhone,” Bachman concludes.

Apple is set to announce its third quarter earnings for the quarter than ended on 30 June on 24 July. There is some speculation that Apple will release Mountain Lion the following day.