The much-anticipated Game of Thrones prequel reportedly has a working title of Bloodmoon.

With the HBO hit set to come to an end next week, The Sun reports that filming for the spin-off series began in Belfast two weeks ago.

The publication suggests that the prequel will follow the story of the Children of the Forest, the original inhabitants of Westeros who first created the White Walkers.

“Thrones fans will be delighted to hear things have started with the prequel,” a source told the paper.

“Those working on the set in Belfast are referring to the series as Bloodmoon. Producers have put together a stellar cast and it will be must-watch telly for anyone who loves Game of Thrones.”

Standard Online has contacted HBO representatives for further comment.

An official synopsis for the spin-off was released last summer, and teased a show that “chronicles the world’s descent from the golden Age of Heroes into its darkest hour.

“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, only one thing is for sure: It’s not the story we think we know.”

Jane Goldman, whose previous credits include Kick Ass, Stardust and Kingsman: The Secret Service, has created the series with Game of Thrones author George R. R. Martin.

Naomi Watts was cast in a “lead role” in the show in October, with Josh Whitehouse, Naomi Ackie, Denise Gough, Jamie Campbell Bower, Miranda Richardson and John Simm also announced.

A release date for the series is yet to be confirmed, though HBO president of programming Casey Bloys has previously stated that the prequels won’t air for “at least a year” after the original series comes to an end.

“I want the final season of Game of Thrones to be the final season of Game of Thrones, I don’t want to use it to launch something else,” he told Deadline last year.

“I want it to stand as the finale of the greatest TV show of all time, I don’t want to do anything that infringes on that.”

The final episode of Game of Thrones will air in the UK on Monday, May 20 at 2am and 9pm on Sky Atlantic and will also be available to stream on NOW TV.