WH Auden attempted to persuade JRR Tolkien to drop the romance between Aragorn and Arwen from the storyline of The Lord of the Rings, describing it as "unnecessary and perfunctory", an unpublished letter by the author has revealed.

The 1955 letter sees Tolkien writing to his publisher about the difficulties of completing The Return of the King, the third and final part of his magnum opus, in which Aragorn and his men face a final battle with Sauron's troops, as the hobbits Frodo and Sam continue on their journey to destroy the One Ring. At the end, Aragorn is crowned king of Gondor, and marries Arwen, the daughter of Elrond, "Evenstar of her people".

"Auden on the whole approves of Vol III seen in galleys," wrote Tolkien, revealing that the poet supported what Tolkien called "the Éowyn-Faramir business", where Éowyn initially falls for Aragorn, but ends up with Faramir when he does not return her love. "Then the heart of Éowyn changed, or else at last she understood it. And suddenly her winter passed, and the sun shone on her," runs the story.

But "he thinks Aragorn-Arwen unnecessary & perfunctory," mourns Tolkien in the letter to Rayner Unwin. "I hope the fragment of the 'saga' will cure him. I still find it poignant: an allegory of naked hope. I hope you do."

Jane Johnson, novelist and former Tolkien publisher, agreed with the fantasy author, describing the Aragorn/Arwen romance as "so poignant – the love of a mortal man for an immortal elven maiden who must sacrifice her immortal heritage if she decided to follow her heart, and who will in any case – since elves are so long-lived – have to suffer seeing her beloved die, leaving her alone with many empty years of her life ahead of her".

"It's a heartbreaking choice: that Arwen chooses to stay in Middle-earth with Aragorn rather than follow the rest of her kind into the Undying Lands demonstrates how fine a man, and king, he is," said Johnson, who also writes the Lord of the Rings and Hobbit film visual companions under her pen-name of Jude Fisher. "It also picks up the theme of the story of Beren and Lúthien – again of a man falling in love with an elven woman – from The Silmarillion, a tale acknowledged by JRR Tolkien to refer to his own romance with his wife, Edith, and engraved on their shared grave."

Auden clearly ended up convinced: in a New York Times review of The Lord of the Rings, published in 1956, he wrote that "the demands made on the writer's powers in an epic as long as The Lord of the Rings are enormous and increase as the tale proceeds – the battles have to get more spectacular, the situations more critical, the adventures more thrilling – but I can only say that Mr Tolkien has proved equal to them".

The letter is due to be auctioned by Bonhams in London on 19 March, when it is expected to fetch between £6,000 and £8,000. It also sees Tolkien agonising about his extensive appendices to the three-volume novel, writing: "I have I expect in the end kept too much. If anything has to be rejected then please do not let it be the Runes and the Table I have painfully arranged (to fit the space I hope and avoid the sad fate of the Feanorian Letters which now look very scrappy!) The last two Family Trees Bolger and Boffin I should be glad to jettison."

He goes on to ask Rayner Unwin, who as a 10-year-old had persuaded his father Stanley to publish The Hobbit, and as an adult convinced him to release The Lord of the Rings: "What do you think. Does this section contain anything it would be sad to lose? I am still very sorry that the 'facsimiles' of the Book of Mazarbul are not in. And of course the name lists which would have given me a chance of providing some Elvish vocabulary … "

"This long and complex letter well illustrates the difficulties Tolkien was experiencing in bringing out this last volume, in his perplexity and lack of self-confidence even offering to ditch the whole appendix on translation and pleading for Rayner's advice," said Bonhams. The letter was given to its current owner's husband by Rayner Unwin on 26th February 1956.

"This letter, which has never been published before, sheds a fascinating light on Tolkien's working methods and the devotion to detail which he lavished on the imaginary worlds of his books," said Matthew Haley, head of Bonhams books department in the UK. "Those worlds clearly felt very real to him and he wanted readers to share that feeling."

The sale will also include a first edition of The Hobbit estimated at £10,000-15,000 and a second edition of The Lord of the Rings in three volumes, each of which has been signed by the author on the title page.