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The creator of the Discworld series was diagnosed with a rare early-onset form of the disease in December.

Pratchett, 59, announced the pledge of about £494,000 at the Alzheimer's Research Trust annual conference.

Telling leading dementia specialists of his determination to find a cure, he said: "I intend to scream and harangue while there is time."

Personally, I'd eat the arse out of a dead mole if it offered a fighting chance

Terry Pratchett

There are 15,000 people in the UK with early-onset dementia, which strikes under the age of 65 years.

Mr Pratchett has a rare form of the disease called posterior cortical atrophy, in which areas at the back of the brain begin to shrink and shrivel.

He says he is starting to notice its effect on him.

"I've given up my driving licence because I didn't feel confident driving. And if I've got something inside out, it's a little bit puzzling getting it the right way round again."

He added: "The curious thing is that writing goes on, although the typing doesn't."

Mr Pratchett is paying for the Alzheimer's drug Aricept because the NHS says he is too young to get it for free.

Lack of funds

The author told the conference he is prepared to go to extreme lengths in order to beat the disease.

He said: "Personally, I'd eat the arse out of a dead mole if it offered a fighting chance.

"I am, along with many others, scrabbling to stay ahead long enough to be there when the cure comes along.

"Say it will be soon - there's nearly as many of us as there are cancer sufferers, and it looks as if the number of people with dementia will double within a generation.

"In most cases, alongside the sufferer you will find a spouse suffering as much.

"It is a shock to find out that funding for Alzheimer's research is just 3% of that to find cancer cures."

In total, an estimated 700,000 people in the UK have Alzheimer's disease.

However, the Alzheimer's Research Trust estimates that just £11 per patient is spent annually on research into the disease - compared with £289 for each cancer patient.

Rebecca Wood, chief executive of the Alzheimer's Research Trust, said the trust currently had to turn down two out of every three research projects due to lack of funds.

She said: "Whilst we were deeply saddened to learn of Mr Pratchett's diagnosis, we are delighted that he has chosen to speak out about his experiences with Alzheimer's disease, to raise awareness about its impact and the desperate need for more research.

"Research is the only way to beat this disease and help people like Terry - to prevent them losing their thinking skills and keep them doing the things they love."