TripAdvisor faces an investigation in Italy over concerns the site is not doing enough to prevent fake reviews.
The Italian competition authority said it had had complaints from consumers and businesses about the website, the BBC reported.
A different probe has been launched to see if agreements Expedia and Booking.com hold with hotels are preventing consumers from getting better deals.
The companies have yet to comment, according to the report.
The Italian competition authority will try to establish whether TripAdvisor has sufficient measures in place to detect reviews made by people who had not visited the place in question.
The watchdog also said it was looking at whether the reviews site did enough to distinguish between content submitted by travellers and posts paid for by hotels and other travel businesses.
Expedia and Booking.com are to be investigated separately over clauses they put in place for hotels that are listed on the sites.
"The analysis centres on clauses applied by Booking and Expedia that prevent hotels from offering better prices and conditions through other online services and, generally, any other booking system (including hotels' own websites)," the watchdog was reported as saying.
"The authority believes the use of such clauses by the main two platforms on the market may significantly limit competition."
The announcement comes in the same week that Italy launched an investigation into Google, Apple, Amazon, and games publisher Gameloft over "free-to-play" games.
The watchdog said it had concerns that the games did not make it clear how much it may cost to progress in games.
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