The untitled series -- which filmed its pilot over the summer -- is not moving forward at the network. ET has reached out to HBO for comment.
The show, created by George R.R. Martin and Jane Goldman (who also served as showrunner), was set to take place "thousands of years before the events of Game of Thrones."
"The series chronicles the world’s descent from the golden Age of Heroes into its darkest hour," the show's logline read. "And only one thing is for sure: from the horrifying secrets of Westeros’ history to the true origin of the white walkers, the mysteries of the East to the Starks of legend… it’s not the story we think we know."
Naomi Watts starred in the pilot as a charismatic socialite hiding a dark secret; Josh Whitehouse, Naomi Ackie, Denise Gough, Jamie Campbell Bower, Sheila Atim, Ivanno Jeremiah, Georgie Henley, Alex Sharp and Toby Regbo also had roles.
ET spoke with Watts about the pressure of joining the Game of Thrones family in September. "I'm trying not to think about it," she admitted. "Yeah, but it is evident how well done it is."
"I had to do some catch up, yeah," Watts added of not knowing GoT when she was approached about the job. "My brother, though, basically said to me, when I told him about it, he said, 'You will die before I let you not do that show. You have to do it.' He is obsessed."
"And, of course, when I did get around to watching it, I just did it all in the space of about, I don't know, two months on a lot of long flights and it's absolutely riveting," she raved. "I loved it."
Game of Thrones aired its series finale in July. In September, it was reported that another prequel series was nearing a pilot order, this one centering on House Targaryen.