The Competition and Markets Authority (CMA) has opened an investigation into hotel booking websites over concerns that they could be in breach of consumer law.
The CMA says it is concerned about the clarity, accuracy and presentation of information of sites such as Expedia and Booking.com, "which could mislead people, stop them finding the best deal and potentially break consumer law".
The regulator said it had written to companies across the hotel search sector to request information so that it could understand more about their business practices.
The probe will focus on how search results generated by such websites are ranked and whether hidden factors – such as the amount of commission a hotel pays the site – has an influence on it.
It will also examine if the discount claims made on sites offer a fair comparison to customers and how they display hidden costs such as taxes or booking fees.
"Around 70% of people who shopped around for hotels last year used these sites and they should all be confident they have chosen the best accommodation for their needs and are getting a good deal," said Andrea Coscelli, chief executive of the CMA.
"Sites need to give their customers information that is clear, accurate and presented in a way that enables people to choose the best deal for them.
"But we are concerned that this is not happening and that the information on sites may in fact be making it difficult for people to make the right choice."
The probe comes two months after the CMA concluded its year-long study of price comparison websites.
The regulator urged consumers to use a variety of price comparison sites to get the best deals for energy, holidays or insurance in its review.