LONDON - Fantasy and science fiction author Terry Pratchett, 59, has been diagnosed with a rare form of early onset Alzheimer's disease, he said in a statement to his fans.

"I would have liked to keep this one quiet for a little while, but because of upcoming conventions and of course the need to keep my publishers informed, it seems to me unfair to withhold the news," Pratchett said.

In the statement posted on the Web site of Paul Kidby, artist for Pratchett's popular Discworld series, Pratchett said the condition was the reason behind a "phantom 'stroke'" he was diagnosed with earlier this year.

Alzheimer's disease is usually found in people over the age of 65. It is characterised by cognitive and motor deterioration, including the loss of memory, muscle mass and language in its latest stages.

Pratchett said he was continuing his work and planned to complete his current commitments. His publisher is HarperCollins.

"Frankly I would prefer it if people kept things cheerful, because I think there's time for at least a few more books yet," he wrote in the statement, dated December 11.

"PS ... This should be interpreted as 'I am not dead'. I will, of course, be dead at some future point, as will everybody else. For me, this may be further off than you think -- it's too soon to tell."

Pratchett was the country's best selling author in the 1990s and has sold more than 45 million books in 33 languages, according to the HarperCollins Web site TerryPratchettBooks.com.

His Discworld series, an often satirical series comprising more than 40 books, depicts a fantasy world shaped like a large disk resting on the backs of four giant elephants which are in turn supported by a giant turtle swimming through space.