Travel website Booking.com has been misleading buyers with pressure-selling tactics, according to a watchdog.
Which? carried out spot checks on six websites as part of a crackdown by the competition regulator the Competition and Markets Authority (CMA).
This was carried out over concerns of pressure selling, misleading discount claims, the impact of commissions on search results and hidden charges.
It is feared that customers could feel rushed into booking a room for more money than they need to pay.
The regulator concluded that practices such as giving a false impression of a room's popularity or not displaying the full cost upfront could potentially break consumer protection law.
It gave the websites - Expedia, Booking.com, Agoda, Hotels.com, ebookers and Trivago - until September 1 to comply.
However Which? found five out of 10 claims of "only one room left" on Booking.com failed to give an accurate picture of availability.
In one example, search results for the Park Plaza Westminster Bridge warned that just one "secret deal" room was available - a superior double room with disability access priced at £232.
However, after clicking through to the booking page, Which? found another 10 superior doubles with internal view available for a cheaper rate of £226.
In total, 34 empty rooms were still available at the same hotel on the same night.
In another example, The Banjo B&B in Liverpool showed "1 room left" - on a budget double room.
When Which? clicked through there were four identical "budget double rooms" for the same price of £49.
In contrast, the five other sites named by the CMA appeared to be complying with the rules, the consumer group said.
Naomi Leach from Which? Travel said: "We found clear evidence that Booking.com has not yet sufficiently cleaned up its act and is flouting the rules on pressure selling, which could lead to millions of consumers being rushed into making a booking.
"It must now provide cast-iron guarantees that it won't continue to mislead holidaymakers with these unscrupulous practices, otherwise the regulator will have to step in with strong action to bring it into line."
A Booking.com spokeswoman said: "We have worked hard to implement the commitments agreed with the CMA and maintain continuing collaboration and dialogue to inform ongoing enhancement of the consumer experience."
