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Mr Jobs wrote in his e-mail: "I am sure all of you saw my letter last week sharing something very personal with the Apple community. 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.

"I have asked Tim Cook to be responsible for Apple's day-to-day operations, and I know he and the rest of the executive management team will do a great job."

In his earlier optimistic letter about his health, Mr Jobs said that the illness had been "robbing me of the proteins my body needs to be healthy".

"Fortunately, after further testing, my doctors think they have found the cause. The remedy for this nutritional problem is relatively simple and straightforward, and I've already begun treatment.

"But, just like I didn't lose this much weight and body mass in a week or a month, my doctors expect it will take me until late this spring to regain it."

Speculation about his health resurfaced in June 2008, when Mr Jobs appeared markedly thinner at an Apple conference.

The share price of Apple was repeatedly hit by rumours about his illness and at one point a news site published his obituary.

Last month Apple announced that Mr Jobs would not give the keynote speech at the Macworld conference, sparking fresh speculation.

Mr Jobs's statement about his health last week was supposed to clarify the situation and reassure investors.

Analysts were split about the consequences of last night's further bulletin.

Tim Ghriskey, chief investment officer with Solaris Asset Management in New York, which holds Apple shares, said: "This certainly doesn't sound good. Steve Jobs is Apple. Having said that, it is a very deep team.

"They've got an amazing product line-up. They've got an amazing pipeline. It is a brand people love and it's certainly not going away.

"Certainly, the company will continue but Steve Jobs has been a visionary and very active in design. Those are big shoes to fill on a long-term basis if he is not able to fill them."

Roger Kay, president and technology market analyst of Endpoint Technologies Associates in Wayland, Massachusetts, said: "Apple has, from the beginning, been unable to be truthful about Steve's condition. The stock price may already reflect some of the uncertainty about his health.

"Apple is more dependent on its CEO than most other companies. Steve is a critical judge of the company's business and that maestro role that he performs is what make Apple great. But there is no maestro-in-training to take over Steve's role."

Apple has tried to highlight other executives in recent months.

Public-speaking duties have been handed to subordinates such as Mr Cook, who introduced its new lap-tops in November, and Jonathan Ive, the British designer of the iPod, who is senior vice-president for design.

The Macworld keynote speech last week was given by Phil Schiller, the head of marketing, who generally was thought to have done a good, if unspectacular, job.

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