The UK's Competition and Markets Authority said it would take firms to court if they did not end unfair and misleading practices.
The watchdog said it had "widespread" concerns about a number of sites, which it has declined to name, including the role commission paid by hotels could influence their rankings and the extent to which they included all costs in the upfront price or introduced them later in the booking.
Pressure selling, involving claims about how many people were looking at the same room, how many rooms were left or how long a deal would be available, could also create a false impression of availability or rush consumers into making a decision, the CMA said.
It also raised concern about the fairness of discount claims made by the sites, such as comparisons with higher prices that were only offered for a brief time.
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