Steve Jobs, the chief executive of Apple, has pulled out of delivering his annual address at Macworld, the convention that has become a highlight of the year for Apple enthusiasts, provoking new speculation about his health.

The company announced that Mr Jobs would not appear at the expo next month for the first time in 12 years. Instead, the presentation will be delivered by Philip Schiller, Apple's senior vice-president for worldwide product marketing.

Apple also said that after the 2009 event, it no longer would participate in Macworld. “It will be Apple's last keynote at the show,” a company statement said. “Apple has been steadily scaling back on trade shows in recent years.”

The news unleashed a fresh bout of speculation among Apple aficionados about Mr Jobs's health, which has been the subject of growing concern in the past few years.

In 2003 he was diagnosed with a rare form of pancreatic cancer. An operation in 2004 year 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 boss is in fine health and that Mr Jobs has repeatedly joked about it. When unveiling a new range of iPods in September he appeared in front of a giant screen carrying the message, “The reports of my death are greatly exaggerated.” This was in response to his obituary file, which had just been updated, being published accidentally by Bloomberg.

Analysts said that, regardless of the truth about the state of Mr Jobs's health, continuing reports about it were having a damaging effect on the company and the technology sector.

Richard Holway, the chairman of Tech Market View, an industry analyst, 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?”

As a result of the news, shares in the company fell by more than 7 per cent to $88.85 yesterday.

Industry insiders also suggested that Apple's move was part of an effort to showcase possible successors to Mr Jobs.

Geoff Blaber, an analyst for 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.”

Apple has certainly tried to highlight other executives in recent months. Public speaking duties have increasingly been handed to subordinates such as Mr Schiller, Timothy D. Cook, Apple's chief operating officer, who introduced the company's new range of laptops in November, and Jonathan Ive, the award-winning British designer of the iPod, who is senior vice-president for design.

Apple executives have said 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 usually roam the stage, discussing and revealing details about new products, which included unveiling 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.

Several large companies, including Adobe, the software maker, and Belkin, the computer accessories manufacturer, have said that they will 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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