Holidaymakers can often get better deals booking directly with travel providers rather than using booking sites, according to Which?
The consumer site found customers had encountered a slew of problems while using booking sites, including struggling to secure cancellations and refunds or encountering poor customer service.
In a survey of nearly 5,000 people, Which? rated booking sites for their prices, transparency of fees, and functionality of their websites - with many brands scoring more poorly this year amid the pandemic.
The results could be handy for anyone hoping to jet off for a winter break once England's lockdown lifts next week.
Although seen as a hassle-free way to book trips abroad, flight booking sites scored particularly badly.
Some companies were impossible to contact for refunds and cancellations, while some charged admin fees that would not have been sought if customers booked through the airline directly.
Netflights had the highest rating in this category, with a score of 65%, despite telling holidaymakers that refunds would take more than 45 days to process.
Meanwhile, Opodo had the lowest score (45%) and Which? has recommended passengers avoid booking with it after it left the Atol scheme.
Comparison sites fared slightly better, with Skyscanner leading the way on 67%.
The only comparison site to receive a five-star rating in any category, it was given top scores for its website's functionality.
Travelocity ranked at the bottom of the table, having only achieved two stars in each category and with a customer service score of 43%.
Rated slightly more favourably were accommodation booking sites, the only category where some sites managed to receive a customer score of more than 70%.
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Airbnb has been recommended by Which? and was still given five stars for its customer service despite dealing with a huge number of cancellations during the UK's lockdown.
Lastminute.com came last in this category for the third year running, scoring three stars across each category.
Which? has recommended that instead of turning to booking sites, anyone hoping to travel should instead try booking directly.
"Booking sites have been seen as a hassle-free way to find the best deal on travel or accommodation when booking a holiday, but our survey highlights serious problems, from dodgy refund policies and unwelcome admin fees, to non-existent customer service," said Rory Boland, editor of Which? Travel.
"This year has shown us that nothing can be guaranteed when it comes to booking a holiday, so it's more important than ever to ensure the company you're booking with can be trusted with your money."
An Opodo spokeswoman said: "While each and every one of our customers is important to us and we strive for 100% customer satisfaction, the number of responses to the Which? survey from Opodo customers (93) is very limited.
"This small sample size is not reflective of the 17 million customers that we have worldwide. We have refunded over one million tickets and 100% of package holidays."
