Best-selling author Terry Pratchett has pulled out of a convention celebrating his novels due to take place in August in Manchester.

Pratchett announced that he had early onset Alzheimer's seven years ago. He made the latest announcement with "great reluctance" on the website of the International Discworld Convention.

 "I have been putting off writing this announcement for quite some time and on good days thought I wouldn't have to write it at all," wrote the author. "I am very sorry about this, but I have been dodging the effects of PCA and have been able to write for much longer than any of us ever thought possible, but now The Embuggerance is finally catching up with me, along with other age-related ailments."

The acronym, PCA, refers to posterior cortical atrophy, a rare form of early onset Alzheimer's from which he suffers.

He told fans on the Discworld convention website on Wednesday that "this is the first time ever that I have been unable to attend a UK convention and I really am very sorry . .They say time marches on, and it does, even though I have been running very fast to keep one step ahead of it. I really was looking forward to seeing your smiley, happy faces. Have fun everyone. Yes, on this occasion, have lots of fun."  

The writer has sold over 75 million copies of his comic fantasy novels, many of which are set in Discworld, a a realm proppd up by four elephants balanced on the back of a huge turtle.