Apple's chief executive, Steve Jobs, has announced that he is to take a medical leave of absence from the company, almost exactly two years after he took a similar one.

In an email to employees, Jobs, who has survived a rare form of pancreatic cancer, said he was taking the break to focus on his health. In late 2008 to mid-2009 Jobs was absent from Apple for six months to have a liver transplant as part of his cancer treatment.

"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," he said in an e-mail.

"I have asked Tim Cook to be responsible for all of Apple's day to day operations."

Cook, who has been at Apple since March 1998, is currently the firm's chief operating officer and has run the company day-to-day before now during previous times when Mr Jobs has been dealing with his health problems.

The news was announced when US stock markets were closed for a public holiday to prevent a precipitous fall in the company's share price. However, Apple shares closed down 6.4 per cent on the Frankfurt stock exchange.