The money saving site warns holidaymakers to keep their wits about them to be sure they’re getting the best deal.

When you search for a hotel via a hotel broker or comparison site, the default results usually show ‘recommended’ or ‘top pick’ hotels, chosen according to the site’s algorithms. Price, location, reviews and other factors relevant to customers usually feed in to this.

But MoneySavingExpert.com (MSE) found that about half of the big hotel booking sites also factor in the commission they receive for bookings in their default rankings.

It claims that a hotel may be ranked higher than similar alternatives if a booking site is paid, or paid more, when a customer makes a reservation.

Eight sites including Booking.com, Expedia and Kayak admit that commission may affect rankings – and all clearly state this to customers on their sites.

Other sites, including Lastminute.com and Skyscanner, told MSE that any commercial agreements have no impact on rankings.

The investigation comes after the Competition and Markets Authority stressed earlier this year that sites should be clear to customers if rankings may be affected by commission. It warned that default search rankings are ‘crucial’ for many when choosing a hotel.

How do different sites’ default results rank hotels?

MSE looked at the default search rankings of some of the hotel booking sites most commonly used by UK customers. It found eight of the 14 it checked consider commercial factors when returning results.

The eight sites are:

• Agoda
• Booking.com
• Ebookers
• Expedia
• Hotels.com
• Kayak
• Trip.com
• Trivago

MSE found that the algorithms used to rank hotels in searches are complicated and differ from site to site.

As multiple factors are typically involved and it’s not always clear which hotels sites are paid to feature, it’s impossible to give specific examples of when individual hotels have been ranked differently. But it’s possible a hotel may be ranked higher than similar alternatives due to the fact it earns the booking site more money.

Guy Anker, deputy editor at MoneySavingExpert.com, said: “Ranking hotels based on commission could be harmful to users, but you could argue that sites are at least being transparent about their practices. Yet don’t see the fact that a hotel booking site could skew its rankings as a reason not to use one. After all, they may still offer the best price.

“But you’ll need to keep your wits about you to find the best deal, so don’t automatically go for the hotel at the top of the search. Before choosing, read reviews, compare prices between sites and the hotel direct, and also check location, booking flexibility, room type and what extras are provided, such as breakfast.”