Shares in Apple were marked lower by investors on Thursday as the company faced a future without its founder and visionary Steve Jobs at the helm.

Jobs resigned as CEO on Wednesday and passed the reins to his right-hand man Tim Cook, saying he could no longer fulfill his duties, but making no mention of the poor health that has forced him to take a back seat this year.

Jobs, who fought and survived a rare form of pancreatic cancer and revolutionized the technology arena with the iPod, iPhone and iPad, is deemed the heart and soul of a company that became the most valuable in the world for a brief period this year.

'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 in a brief letter announcing his resignation.

The letter and a separate statement from Apple raised more questions than it answered about Jobs' health and the future of the company. Jobs will continue as chairman.

While it is unlikely that his departure as CEO will derail Apple's ambitious product-launch roadmap in the near term, there are concerns about whether the company will be as creative without its founder and visionary at the helm.

Apple's stock traded down 4.1 per cent in Frankfurt on Thursday, following a drop of as much as 7 per cent in U.S. after-hours trading when Jobs' departure was announced.

In the company statement, Apple co-lead director Art Levinson on behalf of the board praised Jobs' 'extraordinary vision and leadership' and said he would continue to serve the company with 'unique insights, creativity and inspiration.'

Jobs' battle with pancreatic cancer, which has stretched over several years, has been of deep concern to Apple fans, investors and the company's board.

Over the past two years, even board members have confided to friends their concern that Jobs, in his quest for privacy, wasn't being forthcoming with directors about the true condition of his health.

Jobs has been on medical leave since January 17, with his duties being filled by Cook, who was chief operating officer.

The 56-year-old Jobs had briefly emerged from his medical leave in March to unveil the latest version of the iPad and later to attend a dinner hosted by President Barack Obama for technology leaders in Silicon Valley.

But his often-gaunt appearance had sparked questions about how bad his illness was, and his ability to continue at Apple.

In each of Jobs' three health-related absences, Cook has taken over the helm.

The 50-year-old Alabama native, a former Compaq executive and an acknowledged master of supply-chain management, remains largely untested in Wall Street's view, however.

One Silicon Valley CEO, who declined to be identified because of the sensitive issues involved, said the tone of Jobs' statement indicated his health might be worse than feared.

The Apple chieftain has earned a reputation for commanding every aspect of operations -- from day-to-day running to broad strategic decisions -- suggesting he would not give up the job if he had a choice.

'It's really sad,' the CEO told Reuters. 'No one is looking at this as a business thing, but as a human thing. No one thinks that Steve is just stepping aside because he just doesn't want to be CEO of Apple anymore.'

'It feels like another shoe is going to drop.'

Brand research company Millward Brown said Apple's brand, which it values at over $153 billion, should remain intact.

'Steve Jobs resignation from Apple is sad for him as it presumably presages more illness. However he has left the Apple Brand in rude health so that the company is still poised for future growth,' global brands director Peter Walshe wrote.

'The future direction is mapped out, the successor is in place (also a designer by background), and consumers rate the brand uniquely 'creative', 'fun' and 'adventurous'.'

While Jobs did not give details on the state of his health, oncologists who have not treated the Apple founder said he could be facing several problems tied to his rare form of pancreatic cancer and subsequent liver transplant.-Reuters