Spot checks by Which? on six websites the CMA earlier this year ordered to make changes to their practices found Booking.com was continuing to flout the rules, with a number of its “only one room left on our site” claims failing to give an accurate picture of availability.

Besides Booking.com, Expedia, Agoda, Hotels.com, ebookers and Trivago were all subject to CMA enforcement action over pressure selling, misleading discount claims, the effect of commission on search results and hidden charges.

The CMA concluded practices giving a false impression of a room’s popularity, its availability and cost could potentially break consumer law and gave the six sites until 1 September to make changes.

Earlier this week, the CMA said 25 booking sites had agreed to change how they display information and have signed up to a new set of principles that comply with consumer protection law.

According to the CMA, “most” of the 25 have already made the necessary changes, although some have requested more time. The CMA said it would “closely monitor” each firm’s progress to ensure changes materialise “in a timely manner”.

Which?’s recent investigation found Booking.com search results for the Park Plaza Westminster Bridge warned just one “secret deal” was available for a superior double room at £232, but after clicking through to the booking page, another 10 doubles were available at a cheaper rate – and there were 34 rooms in total still available at the same hotel the same night.

Elsewhere, via Booking.com, The Banjo B&B in Liverpool showed one room left – a budget double. When Which? clicked through, there were four identical “budget double rooms” for the same price of £49.

By contrast, Which? found the other five sites named and shamed earlier this year have made changes. It acknowledged Booking.com had also taken action, but this had not gone far enough. It has called for the regulator to take “strong action” to bring the site in line.

Which? Travel’s Naomi Leach said: “We found clear evidence 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 spokesperson told Which? 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.

The spokesperson added: “The CMA closed its investigation into Booking.com without any finding or admission of an infringement, which we were very pleased with.”