The Advertising Standards Authority (ASA) has opened a formal investigation into the company after receiving a complaint that fake and misleading reviews have reached "epidemic levels," The Times reported. TripAdvisor, which claims to provide "honest travel reviews and opinions from real travelers around the world," receives 45 million visitors every month and a good rating can be worth tens of thousands of pounds in bookings.

But industry experts have warned that large numbers of the site's 50 million reviews are written by hotel owners, agencies acting on their behalf or rivals looking to take their business. In June, an investigation by The Times revealed that hotels were paying agencies to boost their rankings on the website and discredit their rivals.

Hotel owners were found to be paying up to (US$17,000) to companies that employ teams of writers to post hundreds of fake reviews.

Chris Emmins, of the online reputation management company Kwikchex, has been studying TripAdvisor for eight months and said he believed as many as 10 million reviews on the site were fake. He said: "Unless they take some action to actually verify reviews it is going to become useless. The sophistication of the agencies that are working posting fake reviews is getting quite extraordinary."

Several websites and freelance writers openly offer to post fake reviews on TripAdvisor and similar websites. One, candidly called postingonlygoodreviews.com, offers to write up to 1000 reviews for just over £900 a month, The Times said. Good Hotel Guide editor Adam Raphael said the ASA's investigation is "overdue." "TripAdvisor is a very important resource and millions of people use it. It should be properly monitored and supervised," he said. 

"The more important TripAdvisor becomes, the more its results get perverted because hotels submit more and more collusive reviews and then their competitors post malicious reviews." An ASA spokesman confirmed it was looking into a complaint which questioned whether "claims by TripAdvisor on their website, that user reviews were genuine and could be trusted, were misleading and could be substantiated."

A spokeswoman for TripAdvisor said they do not comment on active regulatory investigations, but added: "We take the authenticity of our reviews very seriously and have numerous methods to ensure the legitimacy of the content on TripAdvisor, including automated site tools, a team of review integrity experts, and our large and passionate community of millions of travelers that help us identify suspicious content."