Terry Pratchett described today how the onset of Alzheimer's disease has started to affect the writing of his best-selling fantasy Discworld series.

The author vowed to carry on writing despite having the disease diagnosed last year but revealed that his spelling had become erratic and his typing was no longer as fast.

Pratchett, 60, said that he was trying to be philosophical and was hopeful that researchers could come up with a means of combating the disease.

“I used to touch type as fast as any journalist does and my spelling was pretty good. Now I hunt and peg and my spelling is erratic," he said.

“I can spell 'transubstantiation' and in the next bit I can't spell 'colour' because it's as if bits of the network are switching on and off.

“I'm lucky in a sense, I have been diagnosed quite early and also have got enough push and enough support to get fully diagnosed, and also my books have made me quite a lot of money so I can afford to take an earlier retirement.

“I shan't. I shall carry on writing but I just will have to do it more carefully.”

Pratchett's Discworld series has sold 55 million copies since the first novel was published in 1983. He was awarded the OBE for services to British literature in 1998.

The author told BBC Breakfast he suspected that something was wrong because he was having “Clapham Junction moments, where it just gets too much”.

He passed a mental test involving questions like “what day is it?” and doctors initially told him he did not have the disease.

But he was eventually diagnosed in November last year. He suffers from a rare form of the disease called posterior cortical atrophy, which refers to shrinkage of the back of the brain.

“There's a very good way of not catching Alzheimer's — that's to die around the age of 30, to die before you get old,” he said.

“The other way now is to try and beat the thing, which is what I'm trying to do, which is why I'm now patron of the Alzheimer's Research Trust.

“I gave them some money and I think it's something we can beat.”

In March he gave half a million pounds to the trust to help to find a cure for the disease.

“We should be aware that it's an illness, it's not some visitation from heaven. This is something that can't be repeated often enough," he said.

“It's not something to be ashamed of. It's not because you've done something wrong.”

He said that researchers looking into the disease “seem to be pretty much on the trail” and in five years' time the situation could be looking “hopeful”.

“My father always used to say you have to be philosophical about things, by which he meant stoical,” he said.

“The future is going to happen whether I'm scared of it or not so I do my best not to be. Around about five o'clock in the morning things might be different but you just have to face it.”

Pratchett is one of at least 15,000 people in the UK who are under the age of 65 with early onset dementia.

The progressive, incurable brain disease is the most common form of dementia. People suffering from the early stages of Alzheimer's can experience memory lapses and have difficulty finding the right words.

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