Steve Jobs, the chief executive and co-founder of Apple, announced yesterday that his health problems are "more complex" than he had previously realised and will take a medical leave of absence until the end of June.

The announcement, delivered by email to employees at the Apple campus in Cupertino, California, is likely to feed some of the worst fears of the company's fans and investors who for weeks have been trying to make sense of conflicting rumours about the health of Mr Jobs, 53, who was treated for pancreatic cancer in 2004. In after-hours trading, shares in Apple, which makes the iPod and iPhone, plunged 10 per cent on the news.

Speculation about the condition of Mr Jobs took off midway through 2008 when his gaunt appearance made clear he was rapidly losing weight. Just last week, however, he issued a statement saying he had been diagnosed with a rare hormone imbalance problem that was "relatively simple and straightforward" to deal with.

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He said last night that the day-to-day running of Apple would pass to its chief operating officer, Tim Cook. "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," he wrote. "In addition, during the past week I have learned that my health-related issues are more complex than I originally thought."

He added: "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."

Analysts last night were already wondering aloud, however, whether Mr Jobs will ever return.