A prequel series to the hit fantasy TV show Game of Thrones has been ditched by cable channel HBO, according to US media reports.

Sources quoted by The Hollywood Reporter said executive producer Jane Goldman emailed the cast with the news.

The reasons behind the decision were not immediately clear. HBO has not commented on the reports.

Oscar nominee Naomi Watts was lined up to star in the series and a pilot episode was filmed over the summer.

Entertainment website Deadline reported that the pilot had not been well received by HBO.

• Game of Thrones prequel: What we knew

The prequel, co-created by Game of Thrones author George RR Martin, was reported to have been set about 5,000 years before events in the original series, which ended its eight-season run earlier this year.

The series was to chronicle “the world’s descent from the golden Age of Heroes into its darkest hour”, HBO said when news of the spin-off was announced.

Game of Thrones has been HBO’s most popular series to date, winning multiple Emmy awards and drawing tens of millions of viewers worldwide.

On Monday, Game of Thrones writers David Benioff and DB Weiss announced that they had left the next Star Wars trilogy, due to launch in 2022. The news came after the pair signed an exclusive deal with Netflix.

“There are only so many hours in the day, we could not do justice to both Star Wars and our Netflix projects,” a statement given to Deadline read.