The consumer site found that customers had experienced a number of issues when using the booking sites, including difficulty getting cancellations and refunds or poor customer service.
In a survey of nearly 5,000 people, Which? noted booking sites for their prices, transparency of fees and the functionality of their websites – with many brands scoring lower this year amid the pandemic.
The results could be useful for anyone hoping to leave for a winter break once England’s lockdown is lifted next week.
Despite being seen as an easy way to book trips abroad, flight booking sites have performed very poorly.
Some companies were impossible to contact for refunds and cancellations, while others charged an administrative fee that would not have been claimed if customers had booked directly through the airline.
Netflights scored the highest in this category, scoring 65%, although it told vacationers that refunds would take longer than 45 days to process.
Meanwhile, Opodo had the lowest score (45%) and which one? recommended that passengers avoid booking with him after leaving the Atol program.
Comparison sites performed slightly better, with Skyscanner leading the way with 67%.
The only comparison site to receive a five-star rating across all categories, it received top scores for its website functionality.
Travelocity ranks at the bottom of the table, scoring just two stars in each category and with a customer service score of 43%.
Accommodation booking sites were rated slightly higher, the only category where some sites managed to achieve a customer score above 70%.
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Airbnb has been recommended by Which? and still received five stars for its customer service despite facing a large number of cancellations during the UK lockdown.
Lastminute.com came last in this category for the third year in a row, earning three stars in each category.
Which? recommended that instead of turning to booking sites, anyone wishing to travel should instead try to book directly.
“Booking sites were seen as a hassle-free way to find the best travel or accommodation deal when booking a vacation, but our survey highlights serious issues, ranging from questionable refund policies and fees administrative junk to non-existent customer service, ”said Rory Boland, editor-in-chief of Which? Trip.
“This year has shown us that nothing can be guaranteed when it comes to booking vacations, so it’s more important than ever to make sure the company you are booking with can trust your money. ”
An Opodo spokesperson said, “While each of our customers is important to us and we strive for 100% customer satisfaction, the number of responses to the Which? Opodo customers (93) is very limited.
“This small sample size does not reflect the 17 million customers we have around the world. We have refunded over a million tickets and 100% of the package holidays. “