Ed Zander has been under pressure

The US mobile phone giant said Mr Zander wished to spend more time with his family. He will also leave the post of chairman next May.

Mr Zander's departure comes after Motorola warned in the summer that it would likely make a loss for 2007.

Analysts say Motorola, which has cut 7,500 jobs this year, is struggling with a weak range of handsets.

Motorola shares closed up 2% at $15.97 in New York, having risen 7% in premarket trade.

'My doing'

Mr Zander insisted that his departure was entirely of his own choice.

I think Motorola's mobile devices business needs to continue to refresh, to improve their product line Analyst Lawrence Harris

"[It was] my date, my doing, my time frame," he told the Reuters news agency.

He added that he had agreed with his wife that he would stay in the chief executive role for only four years.

Mr Zander is being replaced by Greg Brown, who is currently Motorola's chief operating officer.

Mr Brown said the company would keep building on the changes it has made, and "continuing on the momentum".

Analyst Lawrence Harris of Oppenheimer said Mr Brown had "clearly done an excellent job" since he was appointed chief operating officer earlier this year.

"I think Motorola's mobile devices business needs to continue to refresh, to improve their product line." said Mr Harris.

"We need more high-end products [from Motorola], more multimedia devices to compete with the likes of the iPhone and Nokia N95.

"I believe we will be seeing improvements."