British author Terry Pratchett poses for a photograph before delivering a petition on behalf of the Alzheimer's Research Trust to 10 Downing Street in London, in this November 26, 2008 file photograph. [Photo/Agencies]

LONDON - Terry Pratchett, the British author whose fantasy novels sold in their tens of millions worldwide, has died of a rare form of Alzheimer's disease aged 66, his publisher said on Thursday.

News about the death of Pratchett - who campaigned during his final illness for legalising assisted death - came on his Twitter account in a series of tweets written in the style of his Discworld novels, where Death always talks in capital letters.

"AT LAST, SIR TERRY, WE MUST WALK TOGETHER," said the first tweet on @terryandrob. "Terry took Death's arm and followed him through the doors and on to the black desert under the endless night," said the second, while a third read simply: "The End".

Pratchett died at home surrounded by his family with his cat sleeping on his bed, Transworld Publishers said. The BBC reported that his publisher had said Pratchett's death was entirely natural and unassisted, despite his campaigning for the right of terminally ill people to be helped to commit suicide.

The author, who wore a trademark broad-brimmed black hat, was diagnosed in 2007 with posterior cortical atrophy, a progressive degenerative condition. Continuing to write, he completed his last book, a new Discworld novel, in the summer of 2014 before succumbing to the final stages of the disease.

Pratchett gave numerous interviews and lectures in which he spoke frankly about his disease - and his love of 16th century English composer Thomas Tallis.

"I would like to die peacefully with Thomas Tallis on my iPod before the disease takes me over and I hope that will not be for quite some time to come, because if I knew that I could die at any time I wanted, then suddenly every day would be as precious as a million pounds," he said in 2010.

"If I knew that I could die, I would live. My life, my death, my choice."