US travel and restaurant website TripAdvisor has failed to do enough to stop fake hotel reviews, British consumer association Which? said in a study published on Friday.
A total 15 out of 100 top-rated hotels around the world displayed TripAdvisor reviews showing "blatant" signs of being fake, the study found.
TripAdvisor responded that it was committed to ensuring accurate information and catching fraudsters, while Which? added that the website had taken down hundreds of reviews after its investigation.
"TripAdvisor's failure to stop fake reviews and take strong action against hotels that abuse the system risks misleading millions of travellers and potentially ruining their holidays," said Naomi Leach, head of the Which? travel division.
She added: "Sites like TripAdvisor must do more to ensure the information on their platforms is reliable, and if they continue to fall short they should be compelled to make changes so holidaymakers are no longer at risk of being duped by a flood of fake reviews."
TripAdvisor spokesman James Kay told the BBC that it would "very aggressively" weed out fake reviews.
"We are doing this more than any other platform out there," he added.
Which? said it had identified problems at hotels in the Middle East, particularly in Cairo and also in Jordan.
And two Las Vegas hotels received almost half of their hundreds of five-star TripAdvisor reviews from first-time reviewers who had never used the site before -- nor used it since.
Friday's Which? study was published as British regulators seek to tackle the sale of fake and misleading online reviews.
The Competition and Markets Authority has already urged social media giant Facebook and auction marketplace eBay to stop the sale of fake reviews on their sites.
In December 2017, hoaxers briefly transformed a garden shed to London's top restaurant on TripAdvisor.
"The Shed at Dulwich", based in a south London back garden, offered invited guests conceptual dishes named after moods, including "Lust - rabbit kidneys on toast", "Empathetic - vegan clams in a clear broth" and "Contemplation - a deconstructed Aberdeen stew".
The non-existant eaterie had worked its way up the rankings thanks to fake reviews sent from different computers to avoid TripAdvisor's defences, but it was rapidly removed.