Stieg Larsson had 'nearly finished' fourth Millennium novel before his death

The Swedish crime writer Stieg Larsson was close to completing a fourth novel to follow on from his bestselling Millennium trilogy in the days before his death, the author's family has confirmed.

There have long been rumours that Larsson – who died of a heart attack in 2004 aged 50, before publication of his debut The Girl With a Dragon Tattoo – had written at least part of a further book in the series which was yet to be published. But whether the novel will ever see the light of day has been thrown into doubt by a bitter dispute between the writer's family and his long-term partner Eva Gabrielsson.

In an interview on American television yesterday, the writer's father Erland Larsson and brother Joakim Larsson both confirmed the existence of a fourth novel.

Joakim told CBS news: "I got the email from Stieg 10 days before he died where he wrote, 'book number four is nearly finished.'" But according to the writer's brother, the book was not the fourth in Stieg Larsson's planned series of 10 titles, and was instead intended to be the fifth novel in the sequence. Larsson had chosen to write it early, "because he thought that was more fun to write than book number four".

Erland Larsson said he had held the manuscript of the book "for a couple of seconds" but that it was now in Gabrielsson's hands.

While Larsson's titles have become a publishing sensation, Gabrielsson has inherited none of the proceeds because, despite living together for over 30 years, she and Stieg Larsson never married. Swedish law does not recognise common-law marriages, so it was his family members who inherited the estate and the rights to the books. Gabrielsson is now engaged in a legal dispute over the money from her partner's work.

Nicky Praca, publicist at Larsson's UK publisher Quercus, said the CBS interview this weekend had come as a surprise. "We knew about a possible fourth manuscript but I don't think we knew it was close to being finished. None of us has seen anything and nothing has been agreed in terms of publishing it," she said. Christopher Maclehose, the editor who signed up Larsson's bestselling trilogy, would only say: "There are more questions than answers."

The three books in Larsson's Millennium series follow the adventures of tattooed young computer hacker Lisbeth Salander and middle-aged journalist Mikael Blomkvist, and have sold millions of copies around the world. The books have already been filmed in Swedish and Daniel Craig and Rooney Mara are set to star in English language versions.

In July, John-Henri Holmberg, a friend of Stieg Larsson's, told the Associated Press that the novelist had revealed the setting of a fourth book, saying it was set in a remote area of northern Canada, Sachs Harbour, with numerous wild animals but fewer than 150 inhabitants.

Whether this Canadian setting was the backdrop to the manuscript for the fifth in the series which the writer was working on, or was intended for the unwritten fourth instalment is shrouded in mystery.