The Oscar-winning scriptwriter praised ITV for resisting the temptation to meddle with the show and allowing him to fully execute his vision.
"ITV wanted us to make our own show. That wouldn't have happened at the BBC," he told the Edinburgh TV Festival. "So I raise a glass to ITV."
Fellowes went on to describe Downton Abbey as "a posh soap opera", making comparisons with one of ITV's other highly-rated serial dramas.
"I live for Corrie," he confessed. "Like them, we tell endless stories about people to whom things happen until the audience doesn't want to see them any more."
Fellowes, who recently completed work on a new drama about the Titanic, has admitted in the past to getting upset by criticism of Downton Abbey.
The show will return to ITV1 later this year.
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