(CNN) — TripAdvisor has defended itself against accusations of misleading travelers by failing to stop some of its highest ranked hotels winning their status with fake reviews.
UK consumer group Which? Travel analyzed a total of almost 250,000 reviews for the 10 top-ranked hotels in 10 global tourist destinations, from Las Vegas to Cape Town. It found that one in seven of the hotels had "blatant hallmarks" of fake reviews, with others raising "serious concerns."
But TripAdvisor called the analysis "flawed" and said its more sophisticated detection tools were able to weed out fraudulent feedback.
Which? Travel said that when it reported 15 of the worst cases to TripAdvisor, the reviews giant -- now one of the world's biggest travel websites -- admitted that all but one of the hotels in question had been caught using fake reviews in the past year.
Six had previously been penalized, and two had received a "red badge warning" -- where TripAdvisor warns users that the hotel is suspected of using fake reviews, and has "repeatedly failed to remedy its behavior and refuses to cooperate with TripAdvisor's investigators."
Yet, Which? Travel said it found that the review patterns had continued unabated despite the measures, and the red badges had been removed.
The group has accused TripAdvisor of a "lack of serious ongoing action to address repeated abuse of the system."
It says that five-star reviews left by new users who have written no other reviews should be a red flag. Its analysis compared the proportion of first-time five-star reviews with that of first-time three-star reviews, which are less likely to be faked.
In total, it studied 247,277 reviews.
"TripAdvisor's failure to stop fake reviews and take strong action against hotels that abuse the system risks misleading millions of travelers and potentially ruining their holidays," said Which? Travel's Naomi Leach.
TripAdvisor has removed hundreds of reviews in light of the investigation, Which? Travel claimed.
"Striking" evidence of fake reviews
The top-ranked hotel in Cairo lost its spot after TripAdvisor deleted suspicious reviews
MARIO GOLDMAN/AFP/AFP/Getty Images
Hotels in the Middle East had "striking" amounts of fake positive reviews, according to the research. TripAdvisor docked 730 five-star reviews of the "best" hotel in Jordan following the analysis -- but the hotel has not been given a red badge. It remains in the top 10 in the country.
At the top-ranking hotel in Cairo, 79% of five-star reviews came from one-time contributors. Just 14% of three-star reviews came from similar accounts.
The hotel has lost its status as a result of the reviews' deletion.
Las Vegas was another hotspot for fake reviews. At two of the top-rated hotels, almost half the five-star reviews came from one-time contributors. For a competitor which did not arouse suspicion, the figure was just 3%.
Some of the top-ranked hotels in London, Paris, Barcelona and Cape Town "gave reason for suspicion," the research showed, but failed to demonstrate the same patterns across the board.
London is among the destinations examined for fake reviews.
Alberto Pezzali/NurPhoto via Getty Images
TripAdvisor called the analysis "based on a flawed understanding of fake review patterns," adding that "it is simply far too simplistic to assume all first-time reviewers are suspicious."
It said that Which? Travel did not have access to IP [internet protocol] information or location data, both of which TripAdvisor uses to detect fake reviews.
"We analyze hundreds of data points about each review -- most of which only we have access to -- and we combine that data with a wealth of knowledge and understanding of review patterns that our team of experts has gained from tracking hundreds of millions of reviews over a near 20-year period," said the company.
"This includes an ability to track and analyze first-time reviews in far more detail and with far more rigor than Which's team was able to do."
Online reviews influence an annual estimated £23 billion ($28 billion) of booking transactions in the UK alone, according to the UK government.
"Sites like TripAdvisor must do more to ensure the information on their platforms is reliable," said Naomi Leach.
"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."