Facebook has been accused of failing to stop the sale of fake reviews through its website more than a month after being urged to act on the issue by the Competition and Markets Authority (CMA).

Consumer group Which? claimed its own investigation had found a number of groups on the social network that were still attempting to recruit people to write fake reviews after the competition watchdog had raised its concerns.

It called on the CMA to now consider taking action against Facebook over the issue.

In June, the CMA urged Facebook and eBay to conduct their own reviews into the problem, saying there was “troubling evidence that there is a thriving marketplace for fake and misleading online reviews”.

Which? said that as part of its investigation it joined 10 of the review groups on Facebook and witnessed more than 55,000 posts being added over a 30-day period, with sellers offering free products in exchange for highly-rated reviews for products listed elsewhere online.

It also claimed to have found a spike in the number of members of such groups on Facebook, which the consumer group suggests could be people moving to new groups as other ones are shut down.

Which? said, in contrast, the situation on eBay had improved significantly but added it had still found one eBay listing advertising five-star reviews for sale, and urged the site to regularly monitor for such posts.

Natalie Hitchins, Which? head of products said: “Our latest findings demonstrate that Facebook has systematically failed to take action while its platform continues to be plagued with fake review groups generating thousands of posts a day.

“It is deeply concerning that the company continues to leave customers exposed to poor quality or unsafe products boosted by misleading and disingenuous reviews.

“Facebook must immediately take steps to not only address the groups that are reported to it, but proactively identify and shut down other groups, and put measures in place to prevent more from appearing in the future.

“The CMA must now consider enforcement action to ensure that more is being done to protect people from being misled online. Which? will be monitoring the situation closely and piling on the pressure to banish these fake review groups.”

It is estimated that more than three quarters of UK internet users consider online reviews when choosing what to buy.

This means billions of pounds of people’s spending is influenced by reviews every year.

Fake and misleading reviews not only lead to people making poorly informed choices and buying the wrong products, but they are also illegal under consumer protection law.

CMA senior director George Lusty said: “It is unacceptable that Facebook groups promoting fake reviews seem to be reappearing. Facebook must take effective steps to deal with this problem by quickly removing the material and stop it from resurfacing.

“This is just the start – we’ll be doing more to tackle fake and misleading online reviews.

“Lots of us rely on reviews when shopping online to decide what to buy. It is important that people are able to trust they are genuine, rather than something someone has been paid to write.”

Facebook said it had removed most of the groups reported to it and was continuing to investigate further.

“We don’t allow people to use Facebook to facilitate or encourage false reviews. We have removed nine of the 10 groups Which? reported to us and are investigating the remaining group,” a company spokesman said.

“We continue to improve our tools to proactively prevent this kind of abuse, including investing in technology and increasing the size of our safety and security team to 30,000."