Which? Travel carried out spot checks on six websites ordered to make changes following enforcement action by the CMA earlier in the year. The sites were accused of pressure selling, publishing misleading discount claims, allowing commissions to impact search results, and having hidden charges.

The CMA says 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, and that it was taking enforcement action against Expedia, Booking.com, Agoda, Hotels.com, ebookers and Trivago, and gave them until 1 September to up their game.

But Which? found Booking.com was still flouting the rules after the deadline passed, with five out of 10 of its “only 1 room left on our site” claims failing 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? scrolled down to find 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.

A spokesperson for Booking.com told Which? that it had worked hard to implement the commitments agreed with the CMA, which include finding ways to inform customers about the data supporting messages on the availability and popularity of specific properties. It also said it maintains ongoing collaboration to further enhance the consumer experience and that the CMA investigation had closed without any finding or admission of an infringement.”

In contrast, the five other sites which were named and shamed by the CMA appeared to have cleaned up their acts on the specific issue of pressure selling.

Which? Travel first highlighted the risk of travellers being misled by these tactics more than three years ago, prompting action from the CMA, which last week announced a further 25 firms, including the likes of TripAdvisor, Airbnb and Google, have signed up to its consumer protection law principles for online booking sites.

These are designed to stamp out practices that could mislead people, stop them finding the best deal and potentially break consumer law.

Naomi Leach, of 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.”