“Cultural work is not factory work,” Eva Gabrielsson insisted this week. Gabrielsson, an architect and the late Stieg Larsson’s partner for 32 years, was outlining her fierce opposition to the plan for a fourth book in the Swedish author’s “Millennium” series of crime novels. She objects to the dilution, for profit, of an existing body of work. “This is also why there are no managers of any artist’s legacy who permit plagiarism of that person’s works: no new paintings ‘in the image of’ Picasso, no new plays using Bertholt Brecht’s characters.”

Larsson’s publishers have said that David Lagercrantz, best known so far for ghosting the memoirs of Swedish football superstar Zlatan Ibrahimovic, will write a fourth Millennium novel.

Due for publication in late 2015, the sequel will resurrect Larsson’s world-conquering double-act – feminist avenger, Lisbeth Salander, and investigative-journalist sidekick, Mikael Blomkvist – for a coda to the trilogy.

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In Stockholm, Eva Gedin of Norstedts Forlag is “proud and excited” that Lagercrantz has taken on “the challenging task of providing Blomkvist and Salander a second life”. In London, Christopher MacLehose of MacLehose Press – who snapped up the trilogy – feels confident that he “will keep a very great storyteller’s flame alive”. Lagercrantz himself has found the work so far “insanely fun”.

But Gabrielsson sees the sequel differently: “In my view, the purpose of the Immaterial Rights Law should be upheld … the artist’s original work may not be subject to changes, additions… which would trample upon the original artist’s intentions.”

The Millennium books, with their addictive alloy of breakneck action sequences, murky conspiracies and searing political critique, have sold more than 75 million copies. They have inspired one Swedish trio of spin-off movies and – so far – one Hollywood re-make of the first volume, The Girl with the Dragon Tattoo, with US-produced versions of the other two in development.

So the contract for a sequel sounds like a one-way ticket to riches and renown. Yet the Larsson legacy (believed to be worth £30m) has given rise to almost as much intrigue as any plot devised by the radical reporter and editor who never saw the cult of his novels spread like a summer bushfire.

He died, aged 50, in November 2004 while climbing the stairs to his office at Expo magazine in Stockholm. Larsson had never married Gabrielsson because he feared that she might become a target for the neo-Nazi groups and other secretive vested interests that he regularly unmasked in Expo.

Shortly after his death, as her 2011 memoir revealed, she laid an ancient Viking curse on all those who had harmed Stieg in his lifetime. She broke a ceramic horse and, uttering imprecations against the “evil ones… who plotted, spied and stirred up prejudice”, threw it into a lake. The original curse specifies a live animal. Nevertheless, the Norse gods seem to have taken note. As Gabrielsson told the Hay Festival last year: “I asked that people would lose their careers, I asked that they would become sick, and that’s what happened. Some even died. It’s horrible.”

Gabrielsson later became embroiled in a long-running dispute over control of the estate of her “soulmate”. A widow in fact but not in law, Gabrielsson could inherit nothing. Sweden’s legal system grants no rights to the unmarried. All revenues from the estate passed to Stieg’s father, Erland, and brother, Joakim.

They set up a company to manage rights and royalties, as the books moved from local splash to global storm. Father and son have said that they offered Gabrielsson £1.75m and a seat on the firm’s board but she – with much of Swedish public opinion behind her – has railed against her formal exclusion from the inheritance.

Contact between the two feuding sides broke down long ago. Gabrielsson summed up the message of the Millennium novels to me by stating that: “The books have a human approach, and clearly state that individual people do matter and may not be abused, lied to, misled or deceived.” Forcefully, she has argued that the estate has not lived up to these ideals.

From the Larsson aficionado’s point of view, the quarrel matters because the writer – who planned 10 books in all – left a 200-page fragment of a fourth Millennium volume behind at his death. Preserved on a laptop, the unfinished narrative takes place in northern Canada. It was intended to deepen Salander’s back-story, and would have been called God’s Revenge. Thanks to the breakdown in relations between Gabrielsson and the estate, this material has never come to light, so it can form no part of Lagercrantz’s sequel.

Previously, Gabrielsson had recalled how outrage at Swedish society’s post-1980s move away from inclusive social democracy fuelled the anger behind the Millennium books. Larsson observed with dismay the rise of “greed, bonus systems, golden parachutes and corruption beyond imagination – all in all a total disrespect for the traditional Swedish values of honesty, equality and the common good”. Now, she sees in the deal for a lucrative sequel another blow to the “open, trusting and respectful” relationships that her partner cherished.