The boss of the Apple computer empire, Steve Jobs, today disclosed that his health problems have become "more complex", prompting him to take extended leave from his role as chief executive until the end of June.

In a shock announcement just eight days after Jobs reassured investors about his well-being, Jobs emailed employees to say that Apple's chief operating officer, Tim Cook, was taking control of day-to-day operations.

Jobs, 53, survived pancreatic cancer five years ago. A deterioration in his health is likely to cause great alarm among both Apple investors and technology enthusiasts, who see him as the driving force behind successful products such as the iPod, the iMac and the iPhone.

In his message to staff, Jobs said: "Unfortunately, the curiosity over my personal health continues to be a distraction not only for me and my family, but everyone else at Apple as well. In addition, during the past week I have learned that my health-related issues are more complex than I originally thought.

"In order to take myself out of the limelight and focus on my health, and to allow everyone at Apple to focus on delivering extraordinary products, I have decided to take a medical leave of absence until the end of June."

Speculation over Jobs' health has been rife since the summer, when he appeared gaunt and drawn at a technology conference, prompting a slide in Apple's stock.

On January 5, Jobs issued a statement acknowledging that he had suffered weight loss. But he blamed his condition on a relatively minor hormone imbalance and said he intended to remain in charge of Apple.

Moments before today's announcement, trading in Apple's shares was suspended on the technology-dominated Nasdaq stockmarket.