Consumers continue to risk being ripped off as a result of fake review “factories” on Facebook that manufacture misleading Amazon product reviews, says Which? It told Guardian Money that “nearly all” of the Facebook groups it uncovered last autumn were still active this month.

Earlier this week it claimed Amazon’s system was being undermined by a flood of fake five-star reviews for unfamiliar brands. Researchers analysed listings of hundreds of popular tech products and found top-rated items were dominated by brands with names such as Itshiny, Vogek and Aitalk, many with thousands of unverified reviews.

In October 2018, Which? said two large Facebook groups, plus some smaller groups, may between them have up to 87,000 members potentially engaged in writing fake reviews.

Inside the Facebook groups, companies post details of products for which they are seeking positive reviews. Reviewers have to pay for the items – so Amazon believes the buyer is genuine – but after leaving a glowing review, the company refunds the purchase price and sometimes pays a fee.

Natalie Hitchins, head of home products and services at Which?, says: “Clearly the current system to prevent this type of activity is not working. People should use independent sites such as Which? to find product reviews they can trust.”

Facebook told Which?: “We don’t allow people to facilitate or encourage the trade of fake user reviews. The groups brought to our attention have now been removed for violating our policies.

“We urge people to continue to use our reporting tools to flag content they think breaks our rules.”

Amazon says it invests “significant resources” to protect the integrity of reviews on its site. “We have clear participation guidelines for both reviewers and selling partners, and we suspend, ban and take legal action on those who violate our policies.”