"The Maze Runner" star had been linked to a leading role in the Amazon Studios series back in September, but he is no longer attached, according to Variety.
The news emerges less than a month after the project, inspired by J.R.R. Tolkien's fantasy books, was renewed for a second season before it has even premiered.
Sources tell Deadline the producers' plan to take a few months' hiatus after filming the initial two episodes for season one to map out the storyline and start the writing process for the follow-up season prompted Poulter's departure, as the 2020 timeline clashed with his other acting commitments.
A replacement for Poulter has yet to be revealed, but actresses Ema Horvath and Markella Kavenagh are said to feature in the cast, alongside former "Game of Thrones" star Joseph Mawle.