A JRR Tolkien expert working on Amazon’s forthcoming multi-series adaptation of The Lord of the Rings has claimed that the retail and streaming giant has been refused permission by the estate to use the bulk of the book’s plot.

In November 2017, Amazon beat Netflix to a $250m (£207m) deal with the Tolkien estate, HarperCollins and Warner Bros to acquire the rights to the fantasy story, and is reportedly spending around $1bn on the adaptation. Details were scarce in the announcement, but Sharon Tal Yguado, Amazon’s head of scripted programming, then promised “a new epic journey in Middle-earth”.

Tolkien scholar Tom Shippey, who is supervising the show’s development, told German fansite Deutsche Tolkien that the estate has refused to allow the series to be set during any period other than the Second Age of Middle-earth. This means Amazon’s adaptation will not cross over at all with events from the Third Age, which were dramatised in Peter Jackson’s Oscar-winning trilogy in which the hobbit Frodo Baggins journeys to destroy the One Ring.

Spanning 3,441 years, the Second Age begins after the banishment of the dark lord Morgoth and ends with the first demise of Sauron, Morgoth’s servant and the primary villain in The Lord of the Rings, at the hands of an alliance of elves and men.

Shippey said that Amazon “has a relatively free hand” to add details since Tolkien did not flesh out every detail of the Second Age in his appendices or Unfinished Tales, a collection of stories published posthumously in 1980. But Shippey called it “a bit of a minefield – you have to tread very carefully”, saying that “the Tolkien estate will insist that the main shape of the Second Age is not altered. Sauron invades Eriador, is forced back by a Númenórean expedition, is returns to Númenor. There he corrupts the Númenóreans and seduces them to break the ban of the Valar. All this, the course of history, must remain the same.

“But you can add new characters and ask a lot of questions, like: What has Sauron done in the meantime? Where was he after Morgoth was defeated? Theoretically, Amazon can answer these questions by inventing the answers, since Tolkien did not describe it. But it must not contradict anything which Tolkien did say. That’s what Amazon has to watch out for. It must be canonical, it is impossible to change the boundaries which Tolkien has created. It is necessary to remain ‘Tolkienian’.”

Contacted by the Guardian, the Tolkien estate declined to confirm or deny Shippey’s claim.

Amazon’s Lord of the Rings series is predicted to start production in 2020. An official release date has yet to be announced, but Shippey revealed that it would span 22 episodes.

• This article was amended on 12 August 2019. An earlier version incorrectly said that Frodo Baggins destroyed the One Ring.