The tale, designed to while away a long car journey, turned into a best-seller.
Yet Richard Adams, who has died aged 96, spent his first 52 years in relative anonymity.
And when he did complete his book, he struggled to find anyone to publish it.
Richard George Adams was  born on 9 May 1920, in Newbury, Berkshire.
He was the son of a country doctor and was brought up in the rolling countryside with views towards the real Watership Down, on the Hampshire border.
One of his earliest memories was seeing a local man pushing a handcart full of dead rabbits down the street.
"It made me realise, in an instant, that rabbits were things and that it was only in a baby's world that they were not."
He suffered the fate of many middle-class boys of the period when he was sent to boarding school at the age of nine, where, by all accounts, he had a miserable time.
He won a scholarship to Worcester College, Oxford, but his education was interrupted by World War Two and he served for five years in the Army before returning to his studies.
He joined the civil service and spent part of his career managing the clean air programme designed to reduce pollution, especially that caused by the many coal fires still burning in British households.
The event that changed his life occurred on a car journey with his family to see Twelfth Night at Stratford-upon-Avon.
His bored children asked for a story and he began telling them a tale about a group of rabbits attempting to escape from their threatened warren.
Adams was persuaded to write it all down, a process that took him more than two years, but he was, at first, unable to find a publisher.
Many of his rejection letters complained that the book was too long and his characters did not fit the common perception of cuddly bunnies.
His rabbits were described with biological realism; they defecated, had sex and engaged in violent battles for dominance.
Eventually, in 1972, after 14 rejections, the publisher Rex Collings saw the potential and agreed to take it on with an initial print run of 2,500 copies.
It was hailed as a children's classic, going on to sell more than 50 million copies, helped along by readings on BBC radio, and a dramatic performance in London's Regent's Park.
Watership Down sold particularly well in the US where canny distributors placed it on the adult publishing list.
On his promotional tours across the Atlantic, Adams played the American idea of the archetypical Englishman, wearing a bowler hat and insisting on English marmalade and mustard wherever he went.
The book, and a subsequent animated film in 1978, became synonymous with rabbits and at least one enterprising butcher advertised: "You've read the book, you've seen the film, now eat the cast."
Inevitably it attracted criticism from some highbrow reviewers. "There is something to be said for myxomatosis," was one caustic comment.
The sudden flow of wealth enabled Adams to retire from the civil service and become a full-time writer.
It also drove him into tax exile on the Isle of Man, although he later returned to his roots in southern England.
By the time Watership Down was published, he was already writing his second book Shardik, the novel he considered his best work.
It is an epic tale of a bear who is a god in an imaginary world and who is abused by the humans in the story.
Shardik did not find favour among critics with some describing it as "preachy", a judgement with which Adams did not disagree.
His commitment to animal welfare was expressed in his third novel, The Plague Dogs, an outspoken attack on animal experimentation.
He admitted that his indignation about vivisection might have got the better of him but the book became another best-seller.
He became president of the RSPCA but his attempts to persuade the charity to adopt a more campaigning stance did not find favour with some of the more conservative members of the ruling council.
He resigned just ahead of a vote which would have severely curtailed his presidential powers.
Despite his campaigning for animals he insisted he was not a sentimentalist.
He refused to condemn a decision to gas rabbits on the real Watership Down in 1998 after their burrows began undermining the hill.
"If I saw a rabbit in my garden I'd shoot it," he once said.
In all, he wrote more than 20 books, including The Girl in a Swing, a ghostly love story with an undercurrent of eroticism, and a prequel to Shardik - entitled Maia - which was criticised for its sexual and sado-masochistic content.
None of these books achieved the success of Watership Down and even a 1997 sequel, Tales from Watership Down, failed to capture the magic of the original.
Richard Adams was essentially a traditional Englishman with a love of the countryside and a belief that, somehow, things were better in the past.
It is perhaps surprising that this natural conservative, from a conventional middle-class background, should have written a book which had such a revolutionary impact on children's literature.
Once upon a time there was a middle-aged civil servant who told his children a story about rabbits.