San Francisco - Shares of Apple opened sharply lower in US trading Thursday as investors worried that the company's performance would flounder without founder and CEO Steve Jobs at the helm.

Jobs announced he was taking medical leave until the end of June to deal with health problems that were more complex than he had earlier divulged. Investors were also concerned that Apple could face shareholder lawsuits alleging that Jobs' halting revelations about his health failed to meet the necessary standards of disclosure.

Apple's shares opened down some 5.7 per cent at 80.50 dollars in morning trading, down from 85.33 dollars a share Wednesday.

Jobs, 53, said Wednesday that he was taking medical leave from the company, leaving Chief Operating Officer Tim Cook in charge.

The news sparked a storm of comment from analysts, bloggers and Apple's legions of fans on the internet. Many wrote that it was unlikely that Jobs would ever return to the company he founded and which he twice guided to the vanguard of the tech revolution.

"The Steve Jobs era is over," wrote Joe Wilcox, the author of Applewatch. "I actually had expected Steve to step down as CEO.

"Instead, he is taking a leave of absence. But the end is the same, with the leave stalling the inevitable. He can't run the company, and I don't expect him to continue as chief executive."

Jobs, who battled pancreatic cancer in 2004, said that his decision was prompted by the fact that his illness was "more complex" than the hormonal imbalance that he just last week said was the reason for his gaunt appearance in recent months.

Apple's success is closely associated with Jobs' leadership and his decision is expected to have a detrimental effect on the company's performance.

"During the past week I have learned that my health-related issues are more complex than I originally thought," said Jobs in the email.

"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."

Jobs said that he would remain involved in "major strategic decisions" during his absence.

Jobs co-founded Apple in 1976, and was instrumental in helping foster the PC revolution by popularizing such technology staples as the mouse and the graphical user interface. However he was forced out of the company after losing a power struggle with the board in
1985, a move which sparked a rapid decline in Apple's fortunes.

He returned to the ailing company in 1997 and proceeded to revitalize it with a focus on design and new technology that yielded such market hits as the iPod, the iPhone and the iMac. He is also the founder of Pixar Animation Studios. (dpa)