Apple shares dropped more than 2 per cent at one point on Monday after the market learnt that Steve Jobs, the charistmatic co-founder and chief executive, had a liver transplant two months ago.

The maker of the iPhone disclosed at the weekend that Mr Jobs was on medical leave until the end of this month, when his return to work would be, at least initially, on a part-time basis. Investors worried about the chief’s long-term health sent Apple stock price down to $136.50 before closing at $137.37, down 1.51 per cent.

Mr Jobs’ gaunt appearance last year was the source of intense speculation. Apple has been secretive about his health since he underwent an operation in 2004 for pancreatic cancer.

In early January, Mr Jobs said that he was suffering from “a hormone imbalance”. Less than a week later, he said that his health issues were “more complex than I originally thought” and that he was taking medical leave for six months, putting Tim Cook, the chief operating officer, in charge of day-to-day operations.

Mr Jobs, 54, has remained in charge of strategy while on leave and the company has remained adamant that he would return to the [helm at the end of June.

Phil Schiller, Apple’s vice-president of marketing, took Mr Jobs’ place this month at a developers’ conference to unveil the group’s latest iPhone. The model sold more than one million units in eight countries in only three days, making it the most successful debut for a smartphone yet.

The launch is the latest in a long line of successes for Apple, whose fortunes have been uniquely linked to Mr Jobs. He returned to the company in 1997 after a 12-year absence to turn around the flagging group with innovative and successful products, such as the iPod and iPhone.

News of his liver transplant has increased talk about the company’s succession plans. Analysts have speculated that Mr Jobs may soon shift to a new role, focusing on big-picture issues and products, leaving Mr Cook to manage the day-to-day running of the company and its 35,000 employees.

Mr Cook, who made his name at Compaq before joining Apple in 1998, is credited with running Apple’s formidable product delivery strategy, sorting out critical details, such as supply chain management. It is thought likely that Mr Cook will take over the post of chief executive at some point, allowing Mr Jobs to take the more strategic role of chairman, directing product development and marketing.

Jeffrey Fidacaro, an analyst at Susquehanna Financial Group, said that some investors would still be worried about Mr Jobs’ health in the long term and whether he could continue as a driving force at Apple. “He’s obviously the clear visionary of the company. There is a concern about his health and what Apple has told us about his health.”

Roger Kay, an industry analyst and president of Endpoint Technologies Associates, said that Mr Cook had played an important role behind the scenes. “If you want to look at Apple’s history and see where they made execution errors and when those ceased, you can time it almost exactly to the arrival of Tim Cook,” Mr Kay said, pointing to several product launches around the late 1990s where Apple would create demand for a new product and then have problems delivering enough of it.

Mr Jobs’ health problems could push him to groom a successor, a task that Mr Kay said the chief executive was unlikely to have undertaken. “You don’t have a little Steve somewhere waiting in the wings. An autocrat like Steve would not allow somebody like Steve anywhere near himself.”

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