Svante Brandén and Anders Lindblom, who grew up with Sweden's best.selling author of all time in the northern Swedish city of Umeå, have blasted Lagercrantz for keeping Larsson's famous characters Lisbeth Salander and Michael Blomqvist alive in a way that makes "a mockery" of the late writer's work.
"Grave robbing were the first words that came into our our heads," wrote the pair in a damning critique of Lagercrantz's new book in Swedish daily Dagens Nyheter on Tuesday.
"The release of the new Millennium book commercializes Stieg's life's work....This whole circus makes a mockery of Stieg, who really deeply disliked our commercialized society."
The friends described the author as a "very compassionate man of great integrity" and criticized the Swedish legal system for failing to protect the copyright of work by dead writers and other artists.
The highly-anticipated fourth instalment of the series which began with Larsson's novel The Girl with the Dragon Tattoo is set for release on August 27th in 35 countries, as The Local reported earlier this year
Its author, David Lagercrantz, is a journalist and writer best known as footballer Zlatan Ibrahimovic's official biographer.
He was not immediately available for comment on Tuesday, but a press spokesperson for his publisher, Norstedts, told The Local that the company remained optimistic about the book's launch, a message echoing that of the firm's director Eva Gedin on Swedish public broadcaster SVT earlier on Tuesday morning.
"I absolutely believe that the response will be positive from readers of Stieg's Millennium trilogy," she told SVT.
"We think this book is important to keep his work alive. We are also being careful to say that this is a sequel and it is the work of David Lagercrantz."
A photo of Stieg Larsson working at the TT news agency in the 1980s. Photo: TT
Earlier this year the 61-year-old said that continuing the wildly successful Millennium trilogy written by her late partner Stieg Larsson was a mistake and should not be happening.
"They say heroes are supposed to live forever. That's a load of crap, this is about money," Gabrielsson told the AFP news agency.
"It's about a publishing house that needs money, (and) a writer who doesn't have anything to write so he copies someone else," she said.
Stieg Larsson died suddenly from a heart attack in 2004 — before the publication and phenomenal success of the dark crime trilogy that took the world by storm.
Svante Brandén and Anders Lindblom also used their debate article in Dagens Nyheter to share some their experiences of Stieg Larsson's early life, suggesting he had long had a troubled relationship with his family, who went on to make billions from his success.
"When we met Stieg at the age of sixteen in the early 70s, he lived in a small basement apartment with a hotplate in the same apartment block as his family. Soon he had a small private one a few houses away."
They wrote that, unlike with other friends, whose parents they knew and met regularly, they were never invited to Stieg's parents home, adding that Stieg was most often found smoking a cigarette in coffee shops around Umeå.
The first three Millennium books have sold more than 75 million copies in more than 30 languages, according to publisher Norstedts.
Just after Larsson's death, his Gabrielsson said in interviews that she had the draft of a fourth book he had begun several months before his death.