Terry Pratchett, the bestselling fantasy author, is to donate USD$1m to help to find a cure for Alzheimer’s.

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

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

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