The antitrust authority said in a statement Monday that TripAdvisor had failed to adopt controls to prevent false reviews, while at the same time promoting the site's content as "authentic and genuine." It has given TripAdvisor 90 days to present a remedy.
The Federalberghi federation of hoteliers welcomed the decision, citing the numerous examples of "defamatory" reviews that have appeared on the site.
TripAdvisor said in a statement that it would appeal the findings, saying it believed its processes, which include a team to detect fraudsters as well as automated tools and algorithms, were "extremely effective in protecting consumers from the small minority of people who try to cheat our system."
"We firmly believe that TripAdvisor is a force for good - both for consumers and the hospitality industry," the company said.
A U.K. regulator has previously said that TripAdvisor must stop claiming that all the reviews on its British site were written by independent travelers, and therefore reliable.