Booking.com, Agoda, Hotels.com and Ebookers were also investigated by the Competition and Markets Authority (CMA) after it raised 'serious concerns' over tactics used in the sector.
The CMA said it will 'do whatever it can' to bring the rest of the online hotel booking sector up to the same standard.
Action was taken last year after concerns practices such as giving a false impression of a room's popularity or not displaying the full cost of a room upfront could mislead people, stop them finding the best deal and potentially break consumer protection law.
Pressure selling: not giving a false impression of the availability or popularity of a hotel or rushing customers into making a booking decision based on incomplete information.
Discount claims: being clearer about discounts and only promoting deals that are actually available at that time.
Hidden charges: displaying all compulsory charges such as taxes, booking or resort fees in the headline price.
Andrew Tyrie, chairman of the CMA, said: "The CMA has taken enforcement action to bring to an end misleading sales tactics, hidden charges and other practices in the online hotel booking market.
"These have been wholly unacceptable."
The CMA stressed that not all of the six sites were guilty of using the tactics but have still committed to the changes.
The watchdog will write to hotel booking sites across the sector - including travel agents, metasearch engines and hotel chains - to explain how they must comply with consumer protection law.