The ITV1 drama, telling the story of the aristocratic Crawley family, is set to return to TV screens later this year for a fifth series, having made its debut in 2010.
But Lord Fellowes has now said in an interview that the saga will not “go on forever”.
He has been commissioned to write a new American drama called The Gilded Age but told The Wall Street Journal that work on the programme would not start until Downton had come to an end.
Lord Fellowes said: “It’s for NBC Universal and it will happen when Downton finishes because I just couldn’t do both at once.
“I haven’t written it yet, but it’s about the old aristocracy, the Winthrops and the Stuyvesants and the new money of oil and gas and shipping in the 1870s. It will all be fiction – it won’t be real people – but when those families descended on New York, they took over.”
The writer, who is also an actor, reassured fans that there would be a fifth series of Downton but referring to a long-running US courtroom drama, added: “I don’t know yet if there is a season six, but it’s not going to go on forever. It won’t be Perry Mason.”
He also speculated that if the Crawleys had been a real family, they would have still been running the estate today.