Steve Jobs has stepped down as CEO of Apple, a stunning move that has left top executive Tim Cook at the helm of the Cupertino-based technology giant.

Jobs, 55, announced his resignation from the company in a letter to its board of directors on Wednesday.

"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," Jobs wrote.

Jobs, a pancreatic cancer survivor, has been on medical leave for an undisclosed condition since Jan. 17.

He co-founded the company in a garage in 1976 -- going on to become an icon of the personal computer industry and, more recently, its shift into mobile devices and the music business.

He left the company once -- forming both Pixar and the short-lived Apple competitor NeXT -- before returning in the late '90s to revive its then-failing fortunes.

Jobs said in his letter he believes "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."

Jobs' health has been a source of speculation and concern on both Wall Street and in Silicon Valley for some time. He is famous for his exacting and hands-on management approach, leaving many with the impression that his well-being is closely tied to that of Apple.

"The street consensus is that about 15 to 20 per cent of the stock price is tied specifically to Steve Jobs and the charisma that comes with him being at the top," noted BNN's Michael Hainsworth, speaking to CTV News Channel.

"It's a huge shift," agreed technology analyst Carmi Levy, "not only for Apple but for the entire industry because both (Jobs and Apple) have been so influential in so many areas."

Jobs did not mention his health or explain why he is unable to continue as CEO in his letter. An Apple spokesperson declined to comment on his health.

Jobs will continue in the new position of company chairman, though there are no immediate plans for his return from leave, the spokesperson told CTVNews.ca.

Cook, previously Apple's COO, has handled day-to-day operations of the company in Jobs' absence. He has filled in for his boss twice before, in 2004 when Jobs was off for cancer treatment and again in 2009 when he had a liver transplant.

"The board has complete confidence that Tim is the right person to be our next CEO," said Art Levinson, chairman of Genentech, on behalf of Apple's board of directors. "Tim's 13 years of service to Apple have been marked by outstanding performance, and he has demonstrated remarkable talent and sound judgment in everything he does."

Levinson added praise for Apple's outgoing leader. "Steve's extraordinary vision and leadership saved Apple and guided it to its position as the world's most innovative and valuable technology company," he said.