The series will be set 300 years before the original, which ended earlier this year after a record-breaking eight seasons.
A Game Of Thrones prequel series has been announced, but there is no word on when it will be released in the UK.
The series will debut on HBO's upcoming streaming service in the US, which launches in May next year and will cost $14.99 (£11.65) a month.
The series - called House Of The Dragon - will begin with 10 episodes and focuses on the House of Targaryen 300 years before the original series.
It was co-created by George RR Martin, whose books the original series is based on.
Speaking when the spin-off was announced, HBO said it would feature "the world's descent from the golden Age of Heroes into its darkest hour".
British actress Emilia Clarke played the role of Daenerys Targaryen in eight seasons of Game Of Thrones but casting for the prequel has not been announced.
The news comes after it was reported that HBO's first prequel of Game of Thrones, starring Naomi Watts, was no longer happening.