TRAVEL website TripAdvisor is under investigation after claims that up to 10 million reviews are actually fakes posted by rival businesses. The move comes after researchers claim to have developed a formula that can identify false reviews on the website with up to 90 per cent accuracy.

The investigation was launched into TripAdvisor by the UK's Advertising Standards Authority following allegations that millions of reviews on the site were fake.

The ASA decided to open a formal investigation into the company after receiving a complaint that fake and misleading reviews had reached "epidemic levels," The Times in London reported.

TripAdvisor claims to provide "honest travel reviews and opinions from real travellers around the world" and receives 45 million visitors every month.

A good rating can be worth tens of thousands of dollars 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.

An investigation by The Times in June 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 $15,260 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.

"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," Mr Emmins said.

Several websites and freelance writers openly offer to post fake reviews on TripAdvisor and similar websites, including one called postingonlygoodreviews.com which offers to write up to 1000 reviews for just over £900 (1373) a month.

Last month researchers developed an algorithm that is able to identify a fake TripAdvisor review by analysing the language used by "guests" providing feedback.

Freelance writers were asked to write 400 fake reviews of hotels in Chicago, US, for the study. They were combined with 400 genuine reviews of the same hotels, and three volunteers were asked to spot the real from the fake reviews but were unable to.

The giveaway signs a review may not be genuine include referring to the hotel by its name and emphasising who the reviewer traveller with. Exclamation marks, frequent use of “I” or “we” and over-using adjectives such as “very” also rang alarm bells.

A spokeswoman for TripAdvisor said it does not comment on active regulatory investigations, but said checks were carried out on all reviews.

"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 travellers that help us identify suspicious content," the spokeswoman said.

Anger about fake reviews has been building for some time. Last year over 700 members of the hospitality business announced they are committing to or considering legal action against the website via Kwikchex.