Casting has begun for a new 'Game of Thrones' spin-off series titled 'House of the Dragon'.

The upcoming prequel to the hit HBO fantasy drama series - which was based on the 'A Song of Fire and Ice' book series by George R.R. Martin - has officially begun hunting for its main cast, according to a report from Entertainment Weekly.

'House of the Dragon' is believed to be based on Martin's book, 'Fire & Blood', and will tell the history of the Targaryen household, which in 'Game of Thrones' includes popular character Daenerys, played by Emilia Clarke.

However, fans hoping to see more of Emilia in 'House of the Dragon' will be disappointed, as EW reports the series is set ''300 years before the events in 'GoT'.''

As of the time of writing little else is known about the project, but it's also believed the show will tackle the Dance of Dragons, which is the Targaryen Civil War that is occasionally referenced in 'GoT', and which ripped apart the fictional land of Westeros.

Earlier this year, it was confirmed 'House of the Dragon' would air ''sometime in 2022''.

HBO executive Casey Bloys said: ''My guess is sometime in 2022. They are starting writing.

''Obviously it's a big, complicated show.''

And the executive also admitted there are no other potential 'GoT' spin-offs in the works at the moment as all focus is on 'House of the Dragon'.

Asked about more possible shows, he said: ''For me for right now, I think getting 'House of the Dragon' on the air will be the number one priority. There are no other blinking green lights or anything like that. Sometime down the road who knows, but there are no immediate plans. We are all focusing on House of the Dragon.''

The show was announced after another spin-off, which was set to take place thousands of years prior to the events of the original series, was dropped by HBO after the pilot episode was shot.

The axed programme had cast Naomi Watts, Josh Whitehouse, Denise Gough, Jamie Campbell Bower, Sheila Atim, Ivanno Jeremiah, Georgie Henley, Alex Sharp, Toby Regbo, Miranda Richardson, Marquis Rodriquez, John Simm, Richard McCabe, and John Heffernan.