British fantasy author Sir Terry Pratchett has died aged 66 after a long battle with Alzheimer's disease, his publisher said today.

He sold more than 85million books worldwide in 37 languages, but also waged a very public struggle with Alzheimer's disease in recent years - and was a vocal campaigner of the right to die.

The author was best known for his satirical fantasy novels set in Discworld, a flat planet resting on the back of four elephants, themselves perched on the back of a vast turtle moving through space.

His death was 'entirely natural and unassisted', it was reported tonight.

Prime Minister David Cameron said he was 'sad to hear' of the writer's death, adding: 'His books fired the imagination of millions and he fearlessly campaigned for dementia awareness.'

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Fantasy author: Sir Terry Pratchett (pictured) has died aged 66 after a long battle with Alzheimer's disease

Couple: Sir Terry with his wife Lyn at the world premiere of his TV film Hogfather in London's Mayfair in 2006

Deputy Prime Minister Nick Clegg added: 'Sad to learn of the death of Sir Terry Pratchett. A fabulous writer, a wonderful imagination and a brave campaigner for dementia awareness.'

Larry Finlay, managing director at Transworld Publishers, said he was 'deeply saddened', adding that Sir Terry 'passed away in his home, with his cat sleeping on his bed, surrounded by his family'.

Sir Terry completed his last book - set like so many of his best-sellers in Discworld - last year.

Mr Finlay said: 'The world has lost one of its brightest, sharpest minds. In over 70 books, Terry enriched the planet like few before him.

'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.

'Terry faced his Alzheimer's disease (an 'embuggerance', as he called it) publicly and bravely. Over the last few years, it was his writing that sustained him. His legacy will endure for decades to come.

'My sympathies go out to Terry's wife Lyn, their daughter Rhianna, to his close friend Rob Wilkins, and to all closest to him.'

Sir Terry's thousands of fans on Twitter were alerted to the news by a series of messages shortly after 3pm.

The messages read: 'AT LAST, SIR TERRY, WE MUST WALK TOGETHER. Terry took Death's arm and followed him through the doors and on to the black desert under the endless night. The End.'

The character of 'death' always speaks in capitals in the Discworld series.

Popular: One of Sir Terry's most famous books was Going Postal, which was turned into a TV series in 2010

Family: Best-selling author Sir Terry is pictured at home with his daughter Rhianna in 1998

Sir Terry's daughter Rhianna later wrote: 'Many thanks for all the kind words about my dad. Those last few tweets were sent with shaking hands and tear-filled eyes.'

The author died peacefully at home after suffering from a rare form of Alzheimer's disease called posterior cortical atrophy (PCA), also known as Benson's syndrome.

The progressive degenerative condition involves the loss and dysfunction of brain cells, and is thought to be behind five per cent of cases of Alzheimer's in the UK.

Writer Neil Gaiman, who collaborated with Sir Terry on the novel Good Omens, said: 'Thirty years and a month ago, a beginning author met a young journalist in a Chinese Restaurant, and the two men became friends, and they wrote a book, and they managed to stay friends despite everything.

'Last night, the author died. There was nobody like him. I was fortunate to have written a book with him, when we were younger, which taught me so much.'

Actor Sir Tony Robinson added: 'Everybody who reads his work would agree his finest creation was his character Death.

'Any fans of his will know Terry in some way has shaken hands with one of his greatest creations.'

Honour: Sir Terry, pictured right after he was knighted by the Queen at Buckingham Palace six years ago

Announcement: Sir Terry's thousands of fans on Twitter were alerted to the news of his death by a series of messages, finishing with: 'The End'. The character of 'death' always speaks in capitals in the Discworld series

'Rest in peace': Penguin Books mourned the loss of one of the 'brightest, sharpest minds' in the world

SIR TERRY AND THE RIGHT TO DIE: 'I HOPE I JUMP BEFORE I'M PUSHED' 'Terry Pratchett - Choosing to Die': Four years ago, Sir Terry was in a documentary about suicide Following his Alzheimer's diagnosis in 2007, Sir Terry launched his right to die campaign, calling for terminally ill people to be allowed to stop fighting if the 'burden gets too great'. The author was a vocal backer of activist Debbie Purdy, a multiple sclerosis sufferer, during her right to die case in 2009, and continued to campaign for those calling for a change in the law. Speaking of his campaign, Sir Terry said: 'I have seen people profess to fear that the existence of a formalised approach to assisted dying could lead to it somehow becoming part of national health policy. 'I very much doubt this could be the case. We are a democracy and no democratic government is going to get anywhere with a policy of even recommended euthanasia. 'If we were ever to end up with such a government, we would be in so much trouble that the problem would become the least of our worries. But neither do I believe in a duty to suffer the worst ravages of terminal illness.' Four years ago, Sir Terry featured in a documentary about suicide in which he followed a man with motor neurone disease to the Dignitas clinic in Switzerland to see him take a lethal dose of barbiturates. Asked why he wanted to make the film, he said it was because he was 'appalled' at the state of the law. He said: 'The Government here has always turned its back on it and I was ashamed that British people had to drag themselves to Switzerland, at considerable cost, in order to get the services that they were hoping for.' Sir Terry even spoke of his own wishes to 'jump before I am pushed' if he ever found himself in a situation where he could not bear illness any longer. Advertisement

Four years ago, Sir Terry featured in a documentary about suicide in which he followed a man with motor neurone disease to the Dignitas clinic in Switzerland to see him take a lethal dose of barbiturates.

Asked why he wanted to make the film, he said it was because he was 'appalled' at the state of the law.

He said: 'The Government here has always turned its back on it and I was ashamed that British people had to drag themselves to Switzerland, at considerable cost, in order to get the services that they were hoping for.'

Hilary Evans - director of Alzheimer's Research UK, for whom he was a patron - said: 'Sir Terry's legacy to dementia research is huge both financially and as an enormous motivation to our supporters and scientists.

'When he announced the inaugural Terry Pratchett research fellowship in 2010, he insisted on a single word to be engraved on the trophy: 'strive'.

'Our scientists continue to act upon this powerful call, and push forwards with the research that will defeat the condition that took Sir Terry from us. We will miss him.'

Sarah Wootton, chief executive of Dignity in Dying, called him 'a committed campaigner' for 'assisted dying'.

She said: 'Sir Terry was fond of saying "It's time we learned to be as good at dying as we are at living" and his brave approach to confronting issues of death, including his own, was a heartfelt demonstration of dignity.'

Several high-profile figures and best-selling authors took to Twitter to pay their respects to the great writer today.

Author Jill Mansell said: 'Oh RIP, Terry Pratchett. I hope the next world is as full of fun and adventure as the ones you created while you were here.'

Booker Prize-winning author Margaret Atwood said: 'Very sad to hear of the death of Terry Pratchett. I vastly enjoy his playful, smart Discworld books.'

Crime writer Val McDermid also tweeted her sympathies following the news. She said: 'The world is a less fantastic place tonight. RIP Terry Pratchett.

'Sympathies to all who loved him. And to us readers who loved his creations. No more Nac Mac Feegles, Sam Vimes et al...'

American actress Mara Wilson, most famous for playing Matilda in the 1996 film, tweeted: 'Crying. Good Omens meant so much to me. I read it while 15 and miserable, my mom read it while sick with cancer. Thank you, #TerryPratchett.'

Health Secretary Jeremy Hunt said: 'Sad to hear of loss of inspirational Alz campaigner Sir Terry Pratchett. His proud legacy: massively more understanding about dementia.'

And film maker Charlie Russell, who made several documentaries about Sir Terry, told BBC News: 'We started filming and he was a lovely man, he was very kind to me to take me into his home and take me into this heart. It was such a pleasure to get to know him.'

With the former PM: Sir Terry (left) talks to Gordon Brown (right) and Sir David Attenborough (2nd left) in 2009

Visit: Sir Terry in 2010 in Kingwell Rise, the small Somerset town, which has Discworld-inspired road names in keeping with the town's official twinning with the fictional city of Ankh-Morpork

Royal appointment: Sir Terry is knighted by The Queen at Buckingham Palace in February 2009

Comedian Ricky Gervais added: ''It's not worth doing something unless someone, somewhere, would much rather you weren't doing it.' RIP the brilliant Terry Pratchett.'

The world has lost one of its brightest, sharpest minds Larry Finlay, Transworld Publishers

Culture Secretary Sajid Javid tweeted: 'RIP Terry Pratchett. Many children owe thanks for his wonderful stories. Thoughts with his family and friends.'

And Alzheimer's Society chief executive Jeremy Hughes said: 'Sir Terry Pratchett fundamentally changed the way dementia is seen and understood.

'Shouting from the rooftops about the absurdity of how little funding dementia research receives, and fighting for good quality dementia care, he was and will remain the truest of champions for people with the condition.'

Several high-profile figures and best-selling authors took to Twitter to pay their respects to the great writer

Sir Terry: The distinctive style of a literary legend

'If cats looked like frogs we'd realise what nasty, cruel little b******* they are', Sir Terry wrote in Lords and Ladies, his 1992 novel based on Shakespeare's Midsummer Night's Dream. 'Style. That's what people remember.'

And it is his distinctive style that people remember. Sir Terry's Discworld series starting with The Colour of Magic in 1983 is 40 volumes of sci-fi fantasy, written with a sharp and wild sense of humour.

'Pratchett is a master storyteller', critic AS Byatt said in 2011. 'He is a master of complex jokes, good bad jokes, good dreadful jokes and a kind of insidious wisdom about human nature (and other forms of alien nature).'

At home: Sir Terry, who has died aged 66, is pictured in his study. His family moved to their £1.5million country manor house in Broad Chalke, Wiltshire, in 1993 and it remained their base

Born Terence David John Pratchett on 28 April 1948, he grew up an only child to David and Eileen Florence Pratchett in Buckinghamshire.

'ENDURING LEGACY': FULL TRIBUTE FROM SIR TERRY'S PUBLISHER It is with immeasurable sadness that we announce that author Sir Terry Pratchett has died at the age of 66. Larry Finlay, MD at Transworld Publishers: 'I was deeply saddened to learn that Sir Terry Pratchett has died. The world has lost one of its brightest, sharpest minds. 'In over 70 books, Terry enriched the planet like few before him. As all who read him know, Discworld was his vehicle to satirize this world: he did so brilliantly, with great skill, enormous humour and constant invention. 'Terry faced his Alzheimer's disease (an 'embuggerance', as he called it) publicly and bravely. Over the last few years, it was his writing that sustained him. His legacy will endure for decades to come. 'My sympathies go out to Terry's wife Lyn, their daughter Rhianna, to his close friend Rob Wilkins, and to all closest to him.' Terry passed away in his home, with his cat sleeping on his bed surrounded by his family on 12th March 2015. Diagnosed with PCA1 in 2007, he battled the progressive disease with his trademark determination and creativity, and continued 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. We ask that the family are left undisturbed at this distressing time. Advertisement

In his early years, he did not know what he wanted to do.

He described himself as a 'nondescript student' and, although he passed the eleven plus, attended High Wycombe Technical High School citing that 'woodwork would be more fun than Latin'.

While his father was an avid member of Chiltern Amateur Radio Club, it was Sir Terry's mother who got him to start reading.

'I wasn't particularly interested in books', Sir Terry admitted in a 2013 interview. 'My mum, God bless her, she rolled up her sleeves and gave me a penny per page, and it worked beautifully.

'I think she only gave me about thruppence, because the third book was The Wind in the Willows.'

He also spoke affectionately about his time as a regional journalist.

After starting at the Bucks Free Press at 16, he proclaimed that 'everyone should have one local journalism job in their lives, especially if they're a nosy parker'.

It was while he was working as a journalist that he interviewed Colin Smythe of Colin Smythe Publishers, who quickly became Sir Terry's agent and helped him find his footing in the literary world.

After various positions in journalism, he became press officer for the Central Electricity Generation Board in 1983, the year in which his first Discworld novel was published.

By the time his fourth Discworld novel was published, the author was able to quit the job at the electricity board and focus on his writing.

Sir Terry married his wife Lyn Purves in October 1968 and she gave birth to their daughter, Rhianna, eight years later.

The family moved to their £1.5million country manor house in Broad Chalke, Wiltshire, in 1993 and it remained their base.

Sir Terry's Discworld series of novels started with The Colour of Magic (right) in 1983, while his work in his later career, such as Snuff (left), continued to sell well

Literary collection: Pyramids (left) and A Last Continent (right) were among Sir Terry's written works

He sold 70million books worldwide with translations into more than 30 languages. In the 1990s he was Britain's best-selling author and, at the turn of the century, was Britain's second most-read author, beaten only by JK Rowling.

SIR TERRY'S BATTLE WITH RARE FORM OF ALZHEIMER'S DISEASE Sir Terry Pratchett suffered from a rare form of Alzheimer's disease called posterior cortical atrophy (PCA), also known as Benson's syndrome. The progressive degenerative condition involves the loss and dysfunction of brain cells, and is thought to be behind five per cent of cases of Alzheimer's in the UK. Although both forms of the disease involve the loss and dysfunction of brain cells, they affect different parts of the brain. Typically, Alzheimer's disease first impacts the sides of the brain - areas which play an important role in memory. In PCA, the disease first affects the back of the brain, known as the occipital lobe. Professor Sebastian Crutch, an expert on Alzheimer's at University College London, said this part of the brain was responsible for vision, so people with PCA often initially experienced visual problems. 'Rather than being based on memory where people forget appointments and where they've put things, for people with PCA the first thing they notice is complex visual behaviour,' he said. 'When they're reading, words will jump around and a lot of people will have difficulty perceiving objects such as glass doors.' The first symptoms of PCA tend to occur when people are in their late 40s, 50s or early 60s, so it has an earlier onset than Alzheimer's disease. But the first signs are often subtle and so it may be some time before a formal diagnosis is made. Advertisement

He was knighted for services to literature in the 2009 New Year Honours and claimed that 'you can't ask a fantasy writer not to want a knighthood. You know, for two pins I'd get myself a horse and a sword'.

Receiving the honour at Buckingham Palace, he said multi-million pound banker bonuses should be spent helping to treat dementia patients.

He also had many other awards to his name, including the British Books Award as the Fantasy and Scientific Author of the Year in 1994.

In addition, he was awarded honorary doctorates of literature by the Universities of Warwick, Portsmouth, Bath and Bristol.

In August 2007, Sir Terry was misdiagnosed as having suffered a stroke, but the following December he announced that he had been newly diagnosed with a very rare form of early-onset Alzheimer's disease which, he said, 'lay behind this year's phantom stroke'.

But he urged people to 'keep things cheerful', adding: 'We are taking it fairly philosophically down here' and predicting that he had time for 'at least a few more books yet'.

Shortly after the diagnosis, Sir Terry donated nearly £500,000 to the Alzheimer's Research Trust. The patchy provision of treatment for thousands of Alzheimer's sufferers made him want to 'kick a politician in the teeth'.

Since then, he's continued to write and his 2011 novel Snuff sold 55,000 copies in the first three days. Perhaps it is because Death is a regular character in the Discworld books that Sir Terry has remained upbeat.

He also became an activist for assisted suicide and presented the BBC's Choosing to Die documentary. He since claimed he would rather die 'sitting in a chair in my own garden with a glass of brandy in my hand'. He always maintained, however, that he did not want to become a 'death fetishist'.

Despite selling more than 85million books, Sir Terry planned to keep Discworld going by passing the flame on to his daughter Rhianna.

In an interview with the New Statesman in 2012, the author said: 'The Discworld is safe in my daughter's hands.'