The Fall of Gondolin, which the author described as "the first real story" set in Middle-earth, is to be published as a stand-alone book for the first time
The author started writing it in 1917, before returning to Middle-earth for The Hobbit and The Lord of the Rings.
And the Tolkien Society say it was probably written during the period he spent at Great Haywood, near Stafford.
HarperCollins made the announcement that The Fall of Gondolin will be published on August 30 this year.
Edited by his son Christopher Tolkien, aged 93, and illustrated by Alan Lee, it will follow the same format as Beren and Lúthien published last year, separating out the story so that it stands alone while showing how the narrative evolved over the years.
Tolkien Society chair, Shaun Gunner, said: "We never dared to dream that we would see this published. The Fall of Gondolin is, to many in the Tolkien community, the Holy Grail of Tolkien texts as one of Tolkien’s three Great Tales alongside The Children of Húrin and Beren and Lúthien.
"This beautiful story captures the rise and fall of a great Elven kingdom, taking place millennia before the events of The Lord of the Rings. This book brings all the existing work together in one place to present the story in full."
This version is the only full account of the story and belongs with ‘the Book of Lost Tales’, the earliest phase of Tolkien’s mythology.
A compressed version of the story was written between 1926 and 1930 to bring it into harmony with the now much changed ‘Silmarillion’.
Much later, in 1951, Tolkien began work on an entirely refashioned account that comes to an abrupt end once Tuor reaches the hidden city of Gondolin.
Christopher Tolkien described his father’s abandonment of this text as “one of the saddest facts in the whole history of incompletion” in The Book of Lost Tales Part Two.
The Fall of Gondolin will run to 304 pages and be published in hardback, deluxe hardback, large print and e-book on August 30.
The book will be published in the USA by Houghton Mifflin Harcourt and in other languages by numerous Tolkien publishers worldwide.
People interested in celebrating the author of The Lord of the Rings’ and his links with Staffordshire can get the chance at a unique exhibition.
This fascinating insight into the early years of JRR Tolkien, one of the world’s best-loved writers, and his time spent in Staffordshire, has already been seen by over 220,000 visitors since it launched in March 2016.
Now the exhibition is currently at Chasewater Country Park until April 21.