The news was confirmed by an HBO executive who also defended the show's controversial violence.
The sixth season is currently being filmed and Michael Lombardo, president of HBO programming, said: "They are feeling two more years after six."
He added that he would like to have more beyond that and would also be open to a prequel to the story of the epic battle for the Iron Throne in the fictional kingdom of Westeros.
The porgramme is based on George RR Martin's A Song Of Ice And Fire books.
The final episode of season five was viewed by over two million people on Sky Atlantic last month.
The show has also received 24 Emmy nominations, including a nod for best drama.
Game Of Thrones has generated controversy for a brutal rape scene and burning of a child but Mr Lombardo defended its graphic depictions as part of the story and said he supported show creators Dan Weiss and David Benioff.
"This show has had violence as part of its many threads from first episode," he said. "There are no two showrunners more careful about not overstepping what they think the line is."
The fifth season also ended by killing off one of its lead characters and fan favorite, Jon Snow, but viewers have been speculating that the character will return - not that Mr Lombardo was giving away anything.
"Everything I've seen, heard and read, Jon Snow is dead," he said.