It will be the first time that Pratchett's phenomenally successful Discworld series, which has sold more than 45 million copies, has been brought to life on the screen by real actors. David Jason will lead the cast as Albert, Death's assistant, a grumpy 65-year-old elf.
If the two-part, four-hour film, which will be shown on Sky One at Christmas, is a success, Pratchett's books could provide an even deeper vein for film-makers than the works of J R R Tolkien and J K Rowling.
Since writing his first Discworld novel in 1983, the author has produced about two books a year. The series now totals 34 volumes, with more on the way.
When Pratchett started writing about a world inhabited by fairies, goblins, elves and humans with special powers, it would have been hard to make a convincing film adaptation. But the advances in special effects demonstrated by the battle scenes in Peter Jackson's Lord of the Rings trilogy mean that it is now possible to recreate his flights of fantasy.
The growing popularity of the Discworld books in America has also helped to create a broader market for a television adaptation. While the series has had a huge fan base in the UK for more than two decades, sales only took off in the US six years ago.
Pratchett described his literary style, which combines fairy-tale elements with mundanely comic moments, as "reality-based fantasy".
Asked why it had taken so long for one of his novels to be filmed, he admitted: "I'm not a very easy person to negotiate with. The books make me a lot of money and I have very much enjoyed writing them. You have to give up a lot of control for the movies and I can't quite bring myself to do it. TV is more fun than movies, because you can get involved more, because these people are close at hand."
Hogfather is about a missing Santa Claus figure. Unless children around the world wake up to presents the following morning, a chain of events will be set in action leading to the destruction of the universe.
Jason, a fan of Pratchett's novels, will take a break from ITV's A Touch of Frost to star in the film.
Ian Richardson, who starred in the Andrew Davies TV adaptation of Michael Dobbs's book House of Cards, will provide the voice of Death, a regular character in Pratchett's fiction, although a 6ft 7in Dutchman will play Death in person.
Pratchett has been closely involved with the feature, which is being shot in Romania because of the need for vast studio space and ready access to mountain scenery.
Johnny and the Bomb, a children's novel by Pratchett, has just been adapted for television on BBC1, starring Zoe Wanamaker, and animated versions of the Discworld novels Soul Music and Wyrd Sisters have been made for Channel 4.
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