Motorola Mobility Holdings Inc warned it would post a quarterly loss again in the current quarter, as it faces new competition from the Apple Inc iPhone, sending its shares down almost 6 percent.

Motorola's most important customer Verizon Wireless, a venture of Verizon Communications Inc and Vodafone Group Plc , starts selling the iPhone in February. Verizon Wireless had been key to the turnaround of Motorola's cellphone business.

Motorola Mobility, which sells television set-top boxes, as well as handsets, expects to post a first-quarter net loss per share of between 9 cents and 21 cents.

In its first quarterly report as a separate company, Motorola said on Wednesday it posted a profit of $80 million, or 27 cents per share compared with a loss of $204 million, or 69 cents per share in the year-ago quarter before it became a stand-alone company.

Revenue rose to $3.4 billion from $2.8 billion in the year ago quarter. About $2.4 billion of its revenue came from the handset business, Motorola said.

Motorola shipped 11.3 million phones in the quarter, including 4.9 million smartphones. The average estimate was for sales of 5.2 million smartphones and total shipments of 10.7 million, according to six analysts contacted by Reuters.

Its shares fell to $32.85 in late trading after closing at $34.83 on the New York Stock Exchange. The stock has already risen 5 percent since January 4, the stock's first official trading day on the New York Stock Exchange.

Motorola Inc split in two in January to form Motorola Mobility and Motorola Solutions Inc .

(Reporting by Sinead Carew; editing by Andre Grenon)