David Gemmell, the popular British author of heroic fantasy, has died suddenly at the age of 57. His writing career began as the result of a misdiagnosis of cancer in the early 1980s. With great energy he threw himself into the task of writing a novel before he died, determined to beat what he believed to be a terminal illness.

He wrote the book with speed and enthusiasm, working habits that soon became established. That first novel was Legend, published in 1984. It was an immediate success, and established Gemmell as the pre-eminent author in this particular genre of fantasy.

With the mistaken diagnosis set aside, and the extraordinary success of this title (the book was later adapted as a graphic novel, and his publishers even renamed their fantasy imprint Legend), Gemmell settled down to producing novels on a regular basis. Over the next two decades he wrote more than 30 long novels. Most of these form part of several series of linked titles, although there are a few, similar, standalone novels. He wrote one conventional thriller, under the pseudonym Ross Harding. He was working on the final book in his Troy series (fantasies of alternative history) when he died.
David Gemmell was born in west London, and had a tough, troubled urban upbringing, reacting against the unwelcome presence in his life of a stepfather. He was expelled from school in his teens, apparently for organising a gambling syndicate. He found work as a labourer, then as a lorry-driver's mate. In the evenings he worked as a nightclub bouncer.

By the time he was in his mid-30s and writing his first novels, Gemmell was a journalist working for a chain of newspapers on the south coast of England, editing three of them. He also worked as a stringer for national newspapers. He wrote fiction during his lunch breaks, but also presumably during company time, because after the third book he was fired. He remained in East Sussex for the rest of his life, freelancing as a writer.

The many thousands of admirers of his books particularly relished his storytelling ability and the quirky, believable characters he created. He was an unabashed entertainer, who made few pretentious claims about his books, all of which are driven by strong narrative, muscular and sometimes didactic prose and a series of violent or difficult encounters, often resolved by physical action or heroics. Gemmell said that he knew and understood violent men, which was why he enjoyed writing about them.

In his novels, typically, a self-doubting but charismatic warrior, together with a group of unlikely companions, overcomes the forces of darkness in a series of violent but often pyrrhic encounters. Gemmell once summed up the content of his novels with these words: "Love, friendship, honour, courage and redemption". He was fascinated by what he saw as the true nature of heroes (he believed them to be unreliably heroic), and this ambivalence runs through almost everything he wrote.

Like all successful novelists, he had his detractors. His critics complained of the repetitive nature of his plots, and the restricted, if colourful, vocabulary. Violent events occur in all his books, and generally provide the sole impetus for plot development. Gemmell was a generic writer within a specialised and somewhat esoteric genre of fiction: the sort of book you might imagine would be classified as "heroic fantasy" was exactly what he wrote, without innovation or noticeable individualism.

The ambivalence extended to his life. At first sight a conservative man, he was an advocate of capital punishment, a devout Christian, a collector of Louis L'Amour westerns, and an admirer of Ronald Reagan, Margaret Thatcher, John Wayne and Sylvester Stallone. But this was only one side of him. His background, through his mother, was socialist. After he became wealthy, Gemmell quietly supported many small charities and good causes. He gave generously to a women's refuge and to a rehab programme for young addicts, and he did much to encourage novice writers. In Hastings, where he made his home, he revived the fortunes and aspirations of the local writers' group, and established a short story prize, administered in Hastings but open to all comers. The Legend competition is still held annually. Gemmell always took an active role in the final judging and award ceremony.

He travelled often, and was visiting Alaska when the first signs of his final illness became apparent. He flew home at once, checked into a private hospital in London and underwent surgery for a quadruple heart bypass. Within two days of the operation he was taking physical exercise, and went home as soon as he was able. He resumed work on his novel, and when he died he was found slumped in front of his computer.

He was a restless and fatalistic man, who enjoyed life and embraced the fruits of material success. He believed he had much more to achieve and would probably have recognised the appropriateness of such a prolific writer dying in harness, his work unfinished.

He is survived by his wife, Stella, and two adult children, Luke and Kate, by his first wife.

David Andrew Gemmell, fantasy novelist, born August 1 1948; died July 28 2006