Steve Jobs, Apple's charismatic chief executive, has pulled out of his annual keynote address at Macworld, the technology show that has become a key event for Apple enthusiasts, provoking new speculation about his health.

The company announced that Mr Jobs will not appear at the event in January for the first time in 12 years. He will be replaced by Philip W. Schiller, Apple’s senior vice president for worldwide product marketing.

Apple also said that after 2009, it would no longer participate in Macworld.

“It will be Apple's last keynote at the show," a company statement said yesterday. "Apple has been steadily scaling back on trade shows in recent years."

The news unleashed a fresh bout of speculation among Apple fans about Mr Jobs’ health, which has been the subject of close scrutiny in the last few years.

He had a rare form of pancreatic cancer diagnosed in 2003. An operation in 2004 was thought to have been successful, but he has looked gaunt and thin at recent public outings, and made shorter appearances than usual.

Apple insists that its chief exectutive is in fine health. Mr Jobs himself joked about the issue as he unveiled the new Apple MacBooks in October, presenting a slide stating that his blood pressure was 110/70.

Analysts said that regardless of their truth, continuing rumours about the state of Mr Jobs’ health were having a damaging effect on the company and the technology sector.

Richard Holway, chairman of Tech Market View, the industry analysts, said: “The trouble is that this has caused all kinds of rumours. Did he pull out because of his health? Is it because they don’t have anything exciting to announce? Is it because Macworld is in trouble and Apple don’t want to be associated with its failure?”

By early evening GMT, shares in the company had fallen by more than 6 per cent following the news

Other industry insiders suggested that Apple’s move was part of an effort to showcase possible successors to Mr Jobs.

Geoff Blaber, an analyst at CCS Insight, said: “Steve Jobs is the main face of Apple. If that company is going to continue to grow, and if their events are going to have the same impact in the future, they have to ensure other people are given the limelight. It’s trying to ensure there isn’t a dependency on one person.”

In recent months, Apple has tried to shine a public spotlight on other executives. Public speaking duties have been handed to subordinates such as Mr Schiller, who will deliver the Macworld address next month, Timothy D. Cook, Apple’s chief operating officer, who introduced Apple’s new range of laptops in November, and Jonathan Ive, the award-winning British designer of the iPod.

Apple executives have stated that they have a succession plan in place but have not discussed it publicly.

Mr Jobs has delivered the keynote address at Macworld since 1997. Over a two-hour period, he would typically roam the stage, discussing and revealing details on new products, which included launching the iPhone at the event two years ago.

The company’s announcement deals a substantial blow to Macworld, which has gained much of its status from Apple's official presence at the show.

Commenting on the announcement, Time magazine wrote that it "was about as shocking as hearing that Barack Obama would be skipping the Inauguration and sending Joe Biden in his stead".

Several other large companies, including Adobe, the software maker, and Belkin, which makes computer accessories, have said they would scale back their presence at the show for financial reasons.

Paul Kent, vice president of IDG, the company that runs Macworld, said: “We look forward to many successful years of Macworld to come.”

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