It will star Paddy Considine and Matt Smith.
Game Of Thrones prequel House Of The Dragon has started production, HBO has said.
The network shared a picture of a socially distanced table read including stars Paddy Considine and Matt Smith.
House Of The Dragon is set 300 years before the events of Game Of Thrones and will tell the story of the Targaryen family.
One of their descendants, Daenerys Targaryen, was a major character in the main series and was played by Emilia Clarke.
The picture shared by HBO featured the cast going over the script while seated at separate tables.
Stars Emma D’Arcy, Steve Toussaint and Olivia Cooke were all at the table read.
Considine, 47, will play the “warm, kind and decent” King Viserys Targaryen in House Of The Dragon.
Smith, 38, will star as Prince Daemon Targaryen.
Fans of George RR Martin’s books will know the Targaryen family becomes embroiled in a deadly civil war.
House Of The Dragon is one of many Game Of Thrones TV projects HBO has in the works, while a stage show for the West End is also in production.
Game Of Thrones was a pop culture phenomenon during its eight-season run but its critically panned finale in 2019 left a sour taste in the mouths of many fans.