The popular drama, which chronicles the lives of the aristocratic Crawley family, debuted in 2010 and has become a global hit.
Meanwhile Lord Fellowes has been commissioned to write a new American drama called The Gilded Age but said 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."
He added: "I don't know yet if there is a season six, but it's not going to go on forever."
He also speculated that if the Crawleys had been a real family, they would have still been running the estate today - albeit in a more "businesslike" way.
"My own belief is that they would survive, but they would be living in a back wing - and you could buy a ticket to visit and they would only come out in the winter," he said.
A special episode of Downton Abbey aired on Christmas Day, showing the family visiting London for debutante Lady Rose's coming out.
Downton Abbey will return to UTV in the autumn.