Apple could be selling 50 million iPhones a year by 2011, according to Bernstein Research analyst Toni Sacconaghi, who has outlined his reasoning in a research note.

He outlines three key factors for Apple to hit this target: overall smartphone market growth; adding more carriers in existing iPhone markets; and selling the device in China.

Specifically, Sacconaghi suggests that if Apple strikes a deal with US operator Verizon Wireless, it could sell 11 million iPhones in 2011. Meanwhile, if its rumoured partnership with China Unicom becomes official, that could be worth 2.9 million sales in the same year.

Meanwhile, the analyst returns to a suggestion he's made before: that Apple should sell lower-cost iPhones.

"We believe that more aggressively targeting the prepaid market with lower price models or offering an iPhone that does not require a data plan - but still has Wi-Fi connectivity - could significantly expand Apple's addressable market," he writes.