British fantasy author Terry Pratchett has died aged 66 after a long and arduous battle with Alzheimer's disease.

The Discworld writer was suffering from posterior cortical atrophy (PCA), a rare form of early Alzheimer’s, which affects the back part of the brain responsible for recognising visual symbols.

"By the time you've reached your sixties you do know that one day you will die and knowing that is at least the beginning of wisdom,” Pratchett told The Telegraph in 2012.

An only child, the best-selling author was born in 1948 in Beaconsfield in Buckinghamshire, England. Pratchett described himself as a "nondescript student" and attibuted his education to the Beaconsfield Public Library. At the age of 13, he published his first short story in a school magazine.

On December 11, 2007 Pratchett posted online that he had been newly diagnosed with a rare form of early-onset Alzheimer's disease. The author suffered from a chest infection earlier this year which became critical in recent weeks.

In April 2008, the BBC began to make a two-part documentary exploring Pratchett's illness. Aired in 2009, the first part of Terry Pratchett: Living With Alzheimers drew 2.6m viewers.  The second instalment drew 1.72m viewers, both statistics a ready testament to his huge popularity among readers.

Pratchett's sales amounted to over 85 million books worldwide in 37 languages. He is the second most-read writer in the UK, and the seventh most-read non-US author in the USA. He finished his final work, a new Discworld novel, last summer before the final stages of Alzheimers took hold.

Transworld, his publishers, have paid tribute to the author, who called his disease an "embuggerance."   "I was deeply saddened to learn that Sir Terry Pratchett has died," Larry Finlay, managing director at Transworld declared. "The world has lost one of its brightest, sharpest minds.

"Terry faced his Alzheimer's disease publicly and bravely. Over the last few years, it was his writing that sustained him. His legacy will endure for decades to come."

The author is survived by his wife Lyn, and their daughter Rhianna. A statement declares: "Terry passed away in his home, with his cat sleeping on his bed surrounded by his family on March 12 2015. Diagnosed with PCA in 2007, he battled the progressive disease with his trademark determination and creativity, and continued to write."