The story will feature younger actors playing Earl and Countess of Grantham, roles played on "Downton Abbey" by Elizabeth McGovern and Hugh Bonneville, the BBC reported.
"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.
"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," the BBC quoted Fellowes as saying at the BAFTA Screenwriters' Lecture series in London.
"I sort of feel there's something quite nice in there because he's a decent cove, and so he feels rather guilty about this which has affected their marriage beyond that."
Fellowes said the spinoff would air after the flagship "Downton Abbey" series wrapped.