Apple Inc. co-founder Steve Jobs has resigned as chief executive officer, saying in a public letter that he can no longer meet his duties in that role.

The company said Jobs will be replaced by Tim Cook, who was Apple's chief operating officer.

It said Jobs, 56, will continue as Apple's chairman.

In the letter, addressed to the Apple board of directors and the "Apple community," Jobs indicated that he had always said he would step down when he could no longer meet his duties and expectations as CEO.

"Unfortunately, that day has come."

Jobs said he believes Apple's best days are ahead of it.

"I have made some of the best friends of my life at Apple, and I thank you all for the many years of being able to work alongside you," the statement said.

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Apple 12-month stock chart

Apple's stock slid seven per cent after hours, Reuters reported. During the day, before the surprise announcement, Apple shares had been little-changed at $376.18 US.

Jobs had previously warned the company and its shareholders that at some point his health might force him to withdraw from active work with the company. A January absence was his third medical leave over several years. He had previously survived pancreatic cancer and received a liver transplant.  

Genentech Inc. chairman Art Levinson, in a statement issued on behalf of Apple's board, said Jobs' "extraordinary vision and leadership saved Apple and guided it to its position as the world's most innovative and valuable technology company."

He said that Jobs will continue to provide "his unique insights, creativity and inspiration," and that the board has "complete confidence" that Cook is the right person to replace him.

Started in a garage with Woz

Jobs' health has long been a concern for Apple investors who see him as an industry oracle who seems to know what consumers want long before they do. Jobs was the driving force behind the iPod, iPhone, iPad and other devices that turned Apple Inc. into one of the world's most powerful companies — this year it became the largest publicly traded company in the world by market capitalization, far surpassing its long-time rival Microsoft and briefly eclipsing oil giant Exxon, which has since jumped ahead again.

LETTER

August 24, 2011

To the Apple Board of Directors and the Apple Community:

I have always said if there ever came a day when I could no longer meet my duties and expectations as Apple’s CEO, I would be the first to let you know. Unfortunately, that day has come.

I hereby resign as CEO of Apple. I would like to serve, if the board sees fit, as chairman of the board, director and Apple employee.

As far as my successor goes, I strongly recommend that we execute our succession plan and name Tim Cook as CEO of Apple.

I believe Apple’s brightest and most innovative days are ahead of it. And I look forward to watching and contributing to its success in a new role.

I have made some of the best friends of my life at Apple, and I thank you all for the many years of being able to work alongside you.

Steve

His own aura seemed part of the attraction. On stage at trade shows and company events in his uniform of jeans, sneakers and black mock-turtlenecks, he'd entrance audiences with new devices, new colours, new software features, building up to a grand finale he'd predictably preface by saying, "One more thing."

Along with fellow Apple co-founder Steve Wozniak and others, Jobs designed and developed the Apple II series of personal computers in the late 1970s. Like Hewlett-Packard founders William R. Hewlett and David Packard, they started in a garage. Jobs went on to lead the creation of the Macintosh, a breakthrough in PC technology.

He resigned from the company in 1984 after a power struggle and left to start his own company, NeXT, which specialized in educational and business platforms. In 1996, he returned to Apple when it bought up NeXT, and he became chief executive officer on Sept. 16, 1997. Apple shares have risen from an adjusted $5.48 on that day to $376.18 at close Wednesday.

Jobs also co-founded and was once the CEO of Pixar Animation Studios, which created such films as  Toy Story, A Bug's Life, Finding Nemo, The Incredibles, Cars and Ratatouille. Pixar merged with The Walt Disney Company in 2006.

Company has had success this year 

The company has succeeded massively this year, exceeding analysts' expectations with its fiscal third-quarter earnings in July. Apple earned $7.79 a share, far above expectations of $5.85 and 55 per cent higher than the company's own profit guidance of $5.03 a share.

Revenue was $28.57 billion in the period, compared with expectations of $25 billion. Jobs said at the time it was their best quarter ever, with revenue up 82 per cent and profits up 125 per cent.

The strong commercial performance was attributed largely to the iPad mobile device. Apple sold 9.25 million iPads and 20.34 million iPhones over the three months.  

With files from The Associated Press