Steve Jobs, the co-founder and head of Apple Inc., the company behind the wildly popular iPhones, iPads, and iPods, has resigned his post.

In a letter dated Wednesday, Jobs told the Apple board of directors and the Apple community he could no longer meet his “duties and expectations” as the company's chief executive officer.

Jobs had battled pancreatic cancer and received a liver transplant. He has been on medical leave since January.

The news sent stock prices quickly tumbling more than five percent in after-hours trading.

Apple said in a statement Jobs had “saved” Apple with “extraordinary vision and leadership.” The company said Jobs has been elected as chairman of the board of directors and will, in that role, continue to contribute “unique insights, creativity and inspiration.”

The statement said Tim Cook will take over as Apple's new top executive at the recommendation of Jobs. Cook had previously served as the company's chief operations officer. Apple's board said Cook has worked for 13 years at Apple and demonstrated “remarkable talent and sound judgment in everything he does.”

Apple is now one of the world's most valuable companies, but in the 1990s, the company nearly went bankrupt. Its fortunes were transformed when it shifted its focus away from being a personal computer manufacturer into a company producing the trendy iPhone and the iPad tablet for audio-visual media.