Booking.com continues to mislead its customers despite a crackdown by regulators, says a new report.
The Competition and Markets Authority has ruled this year that booking sites should review the way they rate and display rooms under selling pressure concerns.
An investigation by Which? says Booking.com is still giving fake accounts of the popularity of the rooms.
However, the site told the BBC that it "worked hard to implement the commitments" agreed with the CMA.
Which? conducted spot checks on six leading hotel booking sites, which were ordered to make changes earlier this year.
Offensive sites had until September to change their practices.
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The sites were previously found to be engaged in practices that included misleading discount orders, pressure selling, and hidden charges.
According to the Consumer Oversight Agency, five out of ten of Booking.com's "only 1 room remaining on our site" claims failed to provide an accurate picture of availability.
Which? claims the site still falsely represents the popularity of the rooms
Banjo B&B in Liverpool showed "1 remaining room" in an "economy double room". But when which? clicked, says there were four identical "double budget rooms" for the same price of £ 49.
The CMA, which began its investigation of online booking sites last October, sent warning letters to "multiple sites" in June, requiring them to review their practices to ensure they were fair and complied with consumer protection laws.
Despite the change, which one? Travel's Naomi Leach says consumers are still under pressure to make reservations.
"We have found clear evidence that Booking.com has not yet sufficiently cleaned up its act and is breaking the rules of selling under pressure, which could lead to millions of hurried consumers making a reservation," she said.
"Now it must provide cast-iron assurances that it will not continue to mislead tourists with these unscrupulous practices. Otherwise, the regulator will have to intervene with strong actions to align."
A Booking.com spokesman told the BBC: "We continually work to bring transparency, choice and value to travelers by constantly testing and improving the way we present our services online.
"We work hard to implement the agreed commitments to the CMA and maintain ongoing collaboration and dialogue to inform the continuous improvement of the customer experience."