Author George R.R. Martin. Photo by Matt Sayles /Invision/AP

George R.R. Martin has confirmed the upcoming 'Game of Thrones' spin-off show will be titled 'The Long Night'.

The 70-year-old writer penned the 'A Song of Ice and Fire' book series that the HBO fantasy show is based on, and has revealed in a casting announcement that the first of the proposed spin-off projects is set to be titled 'The Long Night'.

Earlier this week, Naomi Watts was confirmed as the first big name attached to the upcoming series - which will be a prequel to the existing show - and George tweeted the news on Tuesday night.

He wrote: "Casting is now underway for THE LONG NIGHT, the first of the GAME OF THRONES successor series to ordered to film. HBO has just announced the first cast member: NAOMI WATTS is coming on board as one of our stars. (sic)"

The main series of 'Game of Thrones' is set to come to an end when its eighth and final season airs next year, and 'The Long Night' is one of a reported five spin-off projects currently in development at HBO.

In a recent blog post, George added: "Meanwhile, there are still a couple of other possible prequels in active development. I can't tell you the subject matter of those projects, no, sorry, wish I could. The readers among you might want to grab a copy of FIRE & BLOOD when it is released on November 20, though. (sic)"

Meanwhile Naomi, 50, was revealed to have landed one of the leading roles in 'The Long Night' by Variety magazine earlier this week.

Whilst her exact character details are being kept under wraps, the publication does report she'll be playing "a charismatic socialite hiding a dark secret."

Variety also reports the show, on the whole, will "chronicle the world's descent from the Golden Age of Heroes into its darkest hour. And only one thing is for sure: 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."