A letter in which author JRR Tolkien debates whether to change a major plot point in The Lord of The Rings is going up for auction.

The note, written to Tolkien's publisher Rayner Unwin in 1955, describes the novelist's troubles while completing the third book in the trilogy, The Return of The King.

Elf princess Arwen (played by Liv Tyler) in The Lord of the Rings.

In the letter, Tolkien reveals poet WH Auden had urged him to cut out the romance between his characters Aragorn and Arwen, played by Viggo Mortensen and Liv Tyler in the film series, as he found the relationship "unnecessary and perfunctory".

The writer then appealed to his publisher, "I still find it (the romance) poignant... I hope you do," before going on to address his concerns over whether the book was too long, adding, "I have I expect in the end kept too much."

Viggo Mortensen as Aragorn in The Return of the King.

The letter will go under the hammer at Bonhams auction house in London on March 19 and is expected to fetch as much as $US12,800 ($A14,200).

WENN