Steve Jobs, who has been on medical leave since Jan. 17, Wednesday resigned as Apple Inc's chief executive officer (CEO). Apple's board of directors has named former chief operating officer (COO) Tim Cook as the company's new CEO.

Apple also announced that Jobs will become chairman of the board and Cook will join the board, effective immediately.

"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 said in a letter to Apple's board of directors.

"I believe Apple's brightest and most innovative days are ahead of it," Jobs wrote in the letter. "And I look forward to watching and contributing to its success in a new role."

The 56-year-old Silicon Valley legend first announced in August 2004 that he had been treated for a rare form of pancreatic cancer. Jobs said at the time that his tumor was diagnosed in time and he had had it removed through surgery.

However, speculations about Jobs' health condition have never stopped since then.

In early Jan. 2009, Jobs took a medical leave from the company for nearly half a year, saying that his health issue was more complex than he had thought. Later in June that year, Jobs received a liver transplant at Methodist University Hospital Transplant Institute in the U.S. state of Tennessee.

However, in January this year, Jobs once again said he is taking another medical leave. This time Jobs did not disclose details about his health conditions.

"Steve's extraordinary vision and leadership saved Apple and guided it to its position as the world's most innovative and valuable technology company," said Art Levinson, chairman of Genentech and an Apple board member, said Wednesday in a statement.

"Steve has made countless contributions to Apple's success, and he has attracted and inspired Apple's immensely creative employees and world class executive team. In his new role as Chairman of the Board, Steve will continue to serve Apple with his unique insights, creativity and inspiration," Levinson noted.