Travel advice website TripAdvisor is being investigated by the advertising watchdog over allegations that not all its reviews are genuine.
A formal complaint was lodged with the Advertising Standards Authority (ASA) over TripAdvisor's claim that it publishes "reviews you can trust".
The allegation is that some of the reviews may not be "real or genuine", an ASA spokesman said.
The complaint was lodged by KwikChex, a brand reputation management company.
TripAdvisor says its website attracts 45 million unique visitors every month and contains more than 50 million reviews.
Many of the reviews are posted by site users and help advise customers about hotels and restaurants.
Chris Emmins, of KwikChex, estimated that between five and 10 million user reviews on TripAdvisor could be fake.
"What we really wanted is to see an impartial authority examine TripAdvisor's claims of trustworthiness.
"We don't want them to reveal company secrets, we just want to know how they verify their reviews.
"Instead of saying 'these are reviews you can trust', they need to put a disclaimer up saying they cannot say whether the reviews are verified or not."
Adam Raphael, editor of the Good Hotel Guide, said the investigation is "long overdue".
"TripAdvisor is wide open to collusive and malicious reviews," he said.
Mr Raphael said he and others had set up bogus accounts to test the website's verification systems.
"We used very over-the-top language and obviously we hadn't been to the hotel but the reviews were published.
"One person I know wrote 127 fake reviews just to prove that the systems don't work," he said.
Customers often threaten hotels with bad reviews on the website to get preferential treatment, according to Mr Raphael.
"It's such an influential site, bad reviews can be extremely damaging," he said.
Angus Struthers, senior director of global communications at TripAdvisor, said: "TripAdvisor does not comment on current or potential regulatory investigations or litigation.
"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.
"We devote thousands of hours each year to battling fraud and improving our fraud detection efforts to ensure the integrity of our content."
A recent study commissioned by the website found that 98% of respondents have found TripAdvisor hotel reviews to be accurate of the actual experience, with 69% stating the reviews were "highly or extremely accurate".
The PhoCusWright study surveyed 3,641 respondents.