Steve Jobs – the man credited with transforming Apple from a computer also-ran to (briefly this month) the largest company in the world – has stepped down as CEO. In a surprise move, the 56-year old published his resignation letter last night, recommending that COO Tim Cook – the man long tipped to be his successor – should replace him. Apple confirmed Cook’s appointment in a follow-up statement and also confirmed that Jobs would stay at the firm as chairman of the board.

Although Jobs has been dogged by health issues in recent years – he took six-months medical leave in 2009 – he gave no specific reasons for his resignation. “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,” he wrote. “Unfortunately, that day has come.”

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

Apple later confirmed its  “succession plan” by appointing Cook. "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," it said in a statement.

Regardless of Cook’s abilities, Jobs departure is likely to create a tricky period for Apple, as the company is so closely tied to its outgoing CEO in the eyes of both investors and the public. According to the Financial Times, Apple’s shares fell more than 5 percent in after-hours trading yesterday in response to the news.

Jobs co-founded Apple in 1976 but was ousted from the firm following a board power struggle in 1984. His return to the helm in 1997 prompted Apple’s revival via a series of ground-breaking products; first the iMac (1999), then later with the iPod (2001) and iPhone (2007).