Novelist Terry Pratchett said today he is donating $1m to fund research into Alzheimer's, the incurable brain disease he was diagnosed with last year.

The best-selling fantasy writer announced in December that he had early-onset dementia, a rare form of the disease. Pratchett said it was "a shock and a shame" that less money was spent on Alzheimer's research than on fighting cancer.

He said the donation of about £500,000 would go to the Alzheimer's Research Trust, a British charity.

"There's nearly as many of us as there are cancer sufferers, and it looks as if the number of people with the disease will double within a generation," Pratchett told the group's annual conference.

He said Alzheimer's was "a nasty disease, surrounded by shadows and small, largely unseen tragedies."

Pratchett is best known for his satirical "Discworld" fantasy saga. More than 55 million copies of his books have been sold around the world.

The writer said he had reacted to his diagnosis with "a sense of loss and abandonment (and a) violently coherent fury that made the Miltonic Lucifer's rage against Heaven seem a bit miffed by comparison."

"That fire still burns," Pratchett said, but reassured fans that he planned to continue writing.