In 1969, a young British journalist returned to London after spending 18 months reporting on the Biafran war. His name was Frederick Forsyth. He was 31 years old and, by his own account, flat broke. Needing money quickly, he did what any self-respecting hack would have done: he wrote a thriller.

Initially entitled The Jackal, it told the story of an unnamed assassin hired to kill President de Gaulle. The novel took Forsyth just 35 days to write. He had no great literary aspirations and certainly no intention of revolutionising an entire genre. Forsyth's heroes were John Buchan and Rider Haggard: he simply wanted to tell a riveting story.

This month marks the 40th anniversary of the novel's publication. It is no exaggeration to say The Day of the Jackal has influenced a generation of thriller writers, from Jack Higgins to Ken Follett, from Tom Clancy to Andy McNab. Before, thrillers were self-evidently works of the imagination. Forsyth changed all that; never before had a popular novelist created a world that seemed indistinguishable from real life. His debut had a documentary sense of realism that all but convinced the public they were reading a work of non-fiction. "Sweeping the country," exclaims the flyleaf of my dog-eared copy from 1971 – "the novel that may not be one!"

How Forsyth managed to achieve all this is a story worth telling. In his mid-20s, he had been posted as a journalist to Paris. De Gaulle had granted Algeria its independence, incurring the wrath of the ultra-right: militants in the Organisation de l'armée secrète (OAS) had vowed to assassinate him. "We were all waiting for the mega-story," Forsyth recalled in a recent interview, "the moment when a sniper got him through the forehead." Of course, the sniper never did, but it gave Forsyth an idea. What if the OAS hired a professional hitman, who was able to penetrate the rings of security around De Gaulle? Forsyth had befriended several of the president's bodyguards; he had even reported from the scene of a failed assassination attempt – an account of this real-life incident opens the novel.

Forsyth had something else in his favour. In Biafra, he'd met many mercenaries, who had taught him about the European underworld: how to obtain a false passport; where to buy a custom-made rifle; how to break a man's neck. All of this knowledge was poured in. Yet the novel was still a risk, not least because the ending was already known – De Gaulle had died in his bed in 1970.

The first four publishers Forsyth sent the manuscript turned it down. A thriller set in France with an unnamed anti-hero who fails in his mission? Forget it. Eventually, one man took a chance. Harold Harris, of Hutchinson, agreed to a modest initial print run of 8,000 copies. "It might just work," he said. Well, it worked. The Day of the Jackal became a word-of-mouth sensation. Within two years, Fred Zinnemann had made a superb film adaptation, with Edward Fox as the Jackal. Hutchinson has lost count of how many millions of copies the book has sold.

"It is a perfect example of the adventure story," says Robert Harris, whose own impeccably researched political thrillers belong in the same tradition. "It is very well written, entirely believable, with this intriguing, enigmatic character at its centre."

The Jackal is the obverse of that other great English assassin – James Bond. Alas, he has also influenced some of society's less attractive elements. A Hebrew translation was found in possession of Yigal Amir, the assassin of Yitzhak Rabin, while Ilich Ramírez Sánchez was nicknamed Carlos the Jackal after a copy of the novel was discovered at his flat. That The Day of the Jackal has become a handbook for maniacs should not be the book's lasting legacy. Few writers can claim to have changed the literary landscape. Forty years ago, a penniless British journalist, unwittingly or not, did just that.