Historian Simon Schama, who teaches at Columbia University, says the costume drama, playing in Britain and on U.S. Public Broadcasting Service affiliates, is full of improbable storylines and historical inaccuracies.
The drama indulges in "cultural necrophilia" and offers nothing but "cliches" about an upper class British family and their servants in a stately country home, he wrote in Newsweek.
"Downton serves up a steaming, silvered tureen of snobbery," he wrote this week.
Schama also criticized the show's stock characters.
"The series is fabulously frocked, and acted, and overacted, and hyper-overacted by all the Usual Suspects in keeping with their allotted roles," he wrote.
Producer Gareth Neame defended "Downton Abbey," saying it was not meant to serve as an historical documentary.
"Downton is a fictional drama," Neame told the BBC. "It is not a history program, but a drama of social satire about a time when relationships, behavior and hierarchy were very different from those we enjoy today."
The program is the most popular British drama on U.S. television, attracting more than 4 million viewers per episode, the BBC said.