Author Terry Pratchett said on Thursday he will donate half a million pounds for research into Alzheimer's disease, the degenerative brain disorder he has had for more than two years.

The 59-year-old creator of the "Discworld" series said the number of people with the disease is expected to double within a generation and more money is needed to help find a cure.

"It's a nasty disease surrounded by shadows and small, largely unseen, tragedies," Pratchett will say in a speech to an Alzheimer's Research Trust conference in Bristol. "People don't know what to say, unless they have had it in the family."

Research into the disease attracts only three percent of the funding spent on finding cures for all the various forms of cancer, he added.

Pratchett, who has sold tens of millions of books around the world, announced in December he has a rare form of the disease.

"Soon after I told the world, my Web site fell over," he said. "I had more than 60,000 messages in the first few hours."

The author said his diagnosis brought on a "violently coherent fury that made the Miltonic Lucifer's rage against Heaven seem a bit miffed by comparison".

Announcing his donation to the Trust, he said: "We need you and you need money. I'm giving you a million dollars. Spend it wisely."

Alzheimer's disease is the most common form of dementia, affecting around 700,000 people in Britain.

Rebecca Wood, chief executive of the Trust, said: "Research is the only way to beat this disease.

"The reality is we are scraping for every penny and have to turn down two out of every three research projects."