The competition watchdog has launched an investigation into hotel comparison websites to make sure consumers are not being ripped off.
The probe will look at practices including pressure selling, the use of hidden charges to hike prices and claims about the discounts available.
Competition and Markets Authority (CMA) chief executive Andrea Coscelli warned the information on websites could be making it more difficult for travellers to make informed choices.
The CMA has written to firms across the sector requiring them to give information about their activities.
The investigation will examine tactics such as how hotels are ranked in online searches and whether results are influenced by the amount of commission a destination pays to the website.
Pressure selling where websites can create a false impression of the number of rooms available are other practices under the microscope.
Hidden charges such as taxes or booking fees is another area of concern.
Mr Coscelli said the investigation would “see whether sites are breaking consumer law” and make sure they “help, not hinder” travellers.
He said: “Around 70 per cent 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.
“In today’s increasingly busy world, sites like this offer real potential to help holiday makers save time and money searching for their ideal getaway.
“To do this, 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.”
Alex Neill, a managing director at consumer group Which?, said: “We found evidence of booking sites using pressure selling tactics and making claims about discount deals that don’t add up.
“If the CMA finds breaches of consumer law, they should take enforcement action.”