Steve Jobs is reported to have returned to work yesterday after the liver transplant that the Apple co-founder and chief executive is said to have undergone two months ago.

Mr Jobs has been on medical leave since January. Apple had said that he was due to return, initially on a part-time basis, at the end of this month. However, Mr Jobs was seen by several people at the Apple headquarters in Cupertino, California yesterday, fuelling speculation that he had returned to the helm earlier than expected.

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.

The Wall Street Journal reported last weekend that Mr Jobs had undergone a liver transplant recently. Yesterday, Apple shares fell more than 2 per cent at one point as markets reacted to the news. Investors worried about the chief’s long-term health sent Apple shares down to $136.50 before closing at $137.37, down 1.5 per cent.

The state of Mr Jobs’s health has been a source of intense speculation since he was treated for a rare form of pancreatic cancer in 2004. Apple, which refused to disclose details on the state of his health while he was on medical leave, has refused to comment.

Apple’s launch of a new version of the iPhone last week has been a success, with Apple saying that it sold more than one million of the iPhone 3G S handsets within three days of its launch, surpassing many analysts' expectations.

Mr Jobs was quoted on an Apple press release about the new iPhone’s strong sales yesterday — his first public comment in months, underscoring rumours of his return. "Customers are voting, and the iPhone is winning," he said.

Analysts said that the market was becoming more comfortable with Apple’s executive team following the new iPhone’s launch, but the uncertainty over Mr Jobs’s health continued to make new investors think twice.

Jeffrey Fidacaro, an analyst at Susquehanna Financial Group, said: "Investors will realise that Apple executed well on the launch in Jobs' absence. 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 and what role he'll be playing when he returns."

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