Image copyright HBO Image caption Any prequel series would have to wait until after Game of Thrones' final series airs in 2019

Game of Thrones could be getting a prequel series, HBO has announced, one of five potential spin-offs from the series.

Book author George RR Martin has created the new series alongside British screenwriter Jane Goldman.

HBO has ordered a pilot episode for the show, set thousands of years before the battles over the Iron Throne.

Executives say any spin-off will not be broadcast until after Game of Thrones' final season in 2019.

If picked up, the prequel will chronicle "the world's descent from the golden Age of Heroes into its darkest hour", HBO said in a statement.

"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."

Image copyright Reuters Image caption The prequel would move away from the wars over the Iron Throne of Westeros

Jane Goldman, who wrote Kick-Ass, X-Men: First Class and the Kingsman films, will serve as a showrunner for the as-yet-unnamed show.

Game of Thrones is HBO's most popular series to date. It has won multiple Emmy awards and drawn tens of millions of viewers worldwide.

The channel has teased what Martin described as "successor shows" in the past, announcing in May 2017 a number of writers who were working on these series.

Other than Goldman, they include Max Borenstein, Brian Helgeland, Carly Wray and Bryan Cogman.

Previous reports have suggested HBO could be willing to throw more than $50m at each season of these new shows.

But HBO's programming president Casey Bloys previously told Hollywood Reporter these series would not focus on the current storyline.

"This story, A Song of Fire and Ice, is done," he said. "There's no revival, reboot, spin-off talk."

So what might a prequel series deal with?

Warning: The section below may contain spoilers.

Age of Heroes?

HBO said the series could start in the "golden Age of Heroes", thousands of years before the events in Game of Thrones.

Don't expect any of the current crop of characters to show up. But their ancient forebears may well make an appearance.

Bran the Builder, founder of the House of Stark, was a hero of the time, as well as Lann the Clever, founder of the House of Lannister - two of the central families involved in the Game of Thrones series.

Image copyright HBO Image caption Charles Dance played Tywin Lannister in Game of Thrones

The White Walkers?

The "true origin of the White Walkers" is another plot point HBO teased in their short statement - the blue-eyed humanoids who constitute a major threat in the more recent series of the show.

George RR Martin's books say the creatures were defeated by the Children of the Forest and the First Men millennia before the events depicted in A Song of Ice and Fire - the book that inspired the Game of Thrones television show.

Could a prequel deal with this ancient war? Or the creation of The Wall, the vast barrier stretching across the entire land of Westeros to keep the White Walkers at bay?

Image copyright EPA Image caption White Walkers live north of The Wall

The Starks of legend?

During the Age of Heroes, House Stark moved to unite the North under a single king.

They faced concerted opposition from another family, House Bolton, who fought the Starks and even flayed their men and wore their skin.

Could the ancestors of current show characters Ramsey Bolton and Ned Stark battle it out in the prequel show?

Image copyright HBO Image caption Jon Snow may not technically be of House Stark - but he was no fan of Ramsay Bolton

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