Steve Jobs is to take another medical leave of absence from his role of CEO at Apple, it has been announced.

In an email to Apple staff Jobs said the medical leave will enable him to focus on his health. Chief operating officer Tim Cook will take over the day to day running, although Jobs will still be involved in major decisions at the Cupertino-based firm.

"At my request, the board of directors has granted me a medical leave of absence so I can focus on my health. I will continue as CEO and be involved in major strategic decisions for the company," the email said. "I have asked Tim Cook to be responsible for all of Apple’s day to day operations. I have great confidence that Tim and the rest of the executive management team will do a terrific job executing the exciting plans we have in place for 2011."

The email said Jobs hopes to be back as soon as possible, but no time frame was given.

Steve Jobs was treated for pancreatic cancer in 2004 and, following dramatic weight loss, it was announced in January 2009 that he was suffering from a hormone imbalance. Shortly after this it was announced Jobs would take a six-month leave of absence. After his return to the top chair at Apple it was revealed he had undergone a liver transplant.

Today is a public holiday in the US so no trading in stocks and shares takes place. However, Apple shares dipped 7% in Frankfurt when the news was announced, according to the BBC.