The first prose piece by Lord of the Rings author JRR Tolkien is to be published in Britain on Thursday, a version of an epic Finnish poem that experts describe as "undeniably his darkest work".

Written in 1914-1915 when Tolkien was still a student at the University of Oxford, The Story of Kullervo shows the young author "finding his feet", Vincent Ferre, professor of comparative literature at University Paris Est-Criteil told AFP.

Fascinated by ancient languages from a young age, Tolkien was taken by the 19th century work of epic poetry, Kalevala, a compilation of mythology and folklore which tells the story of Kullervo.

The story is "the first time that JRR Tolkien, who had been a poet until then, began writing prose," Ferre, a Tolkien expert, said.

"We could say that Tolkien is finding his feet. He ultimately leaves the story to one side without finishing it, switching to write more personal and original works," he added.

The Story of Kullervo was first published in 2010 in the academic journal Tolkien Studies by English professor Verlyn Flieger.

Flieger had copied out a manuscript written by Tolkien in pencil held at the University of Oxford's Bodleian Library.

"I began to think that it deserved a wider audience than subscribers to a scholarly journal," Flieger told AFP.

After some consideration, the Tolkien estate granted permission.

"It's his earliest mythic story, and thus a precursor of all that is to come," said Flieger, who edited the edition.

"It is also undeniably his darkest work, and thus foreshadows the darker and more sombre aspects of his invented world."

Kullervo is a precursor to Turin Turambar, an important character in Tolkien's works who is the protagonist of the novel The Children of Hurin and appears in mythological saga The Simarillion.

Both Kullervo and Turambar are cursed, and have a father who was the tragic victim of a powerful figure with magical abilities, according to Ferre.

The Simarillion was published four years after Tolkien's death in 1973 and several other works followed, including The Children of Hurin in 2007, The Legend of Sigurd and Gundrun in 2009, and The Fall of Arthur, an unfinished poem published in 2013.

According to Flieger, much unpublished writing by Tolkien is kept at the Bodleian, mainly "lectures and lecture notes, as well as shorter writings".

The author's son Christopher Tolkien had released the best material in the Tolkien archive, including middle-earth, Nordic and Arthurian texts, and the "extraordinary" lectures and translation of Old English poem Beowulf, Ferre said.

"Clearly there remain hundreds of pages of work by Tolkien that are still unknown to the general public, especially concerning his invented languages."