Fellowes explained at the Bafta Screenwriting Lecture that he would like to focus on the beginning of the relationship between Cora and Robert, according to The Daily Telegraph.
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He added that he originally planned the spin-off as a novel, but that ITV would adapt it for television.
"I do actually have an idea of doing a prequel of the courtship of Robert and Cora, when all those American heiresses were arriving in London - the Buccaneers, as they were called," Fellowes explained.
"They had a slightly troubled courtship, because she was in love with him before they married, as we know, and he married her entirely for her money. I sort of feel there's something quite nice in there because he's a decent [man], and so he feels rather guilty about this."
Robert and Cora are played in the original Downton Abbey series by Hugh Bonneville and Elizabeth McGovern, although a pair of young actors would take on the roles for the prequel.
However, Fellowes revealed that he does not want to air the prequel until Downton Abbey has ended, saying: "I don't think you can continue a narrative in more than one area at once."
Downton Abbey airs on Sundays at 9pm on ITV1.
