British science fiction and fantasy author Terry Pratchett, whose Discworld novels have sold tens of millions of copies worldwide, has died at the age of 66 after a long fight with a degenerative brain condition.

The author reportedly died at his home on Thursday surrounded by his family and his pet cat, his publisher said.

“The world has lost one of its brightest, sharpest minds,” said Transworld Publishers managing director Larry Finlay in a statement.

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In 2007, Pratchett was diagnosed with a rare form of early-onset Alzheimer’s disease, which he called an “embuggerance”, and became a campaigner to raise awareness of the disease.

He was also a patron of assisted suicide campaign group Dignity in Dying.

Pratchett was famed worldwide as the creator of the Discworld novels about a flat, parallel universe balanced on the back of four elephants which themselves stand on the shell of a giant turtle.

The first book in the series, “The Colour of Magic”, was published in 1983 and he completed the 41st last summer, before succumbing to the final stages of the Alzheimer’s.

“As all who read him know, Discworld was his vehicle to satirise this world: he did so brilliantly, with great skill, enormous humour and constant invention,” Finlay said.

In total Pratchett wrote more than 70 books over the course of his career.

“Terry enriched the planet like few before him,” Finlay said, adding: “His legacy will endure for decades to come.”

Pratchett is survived by his wife Lyn and daughter Rhianna.