Fake review factories that run on Facebook and manufacture misleading five-star reviews that are then posted on Amazon have been uncovered by investigators from Which?

The consumer group said two large Facebook groups – Amazon Deals Group and Amazon UK Reviewers – were behind the unscrupulous practice, along with smaller groups. Together they may 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. The reviewers have to pay for the items – so that Amazon believes the buyer is genuine – but after leaving a glowing review, the company refunds the purchase price through PayPal, and sometimes pays an additional fee.

Undercover researchers for Which? set up dedicated Amazon and Facebook accounts and requested to join several of the “rewards for reviews” groups.

“They were instructed to order a specified item through Amazon, write a review and share a link to the review once it was published. Following the successful publication of the review, a refund for the cost of the item would then be paid via PayPal,” said Which?

But the Which? investigators turned the tables on the fake review factories by posting their honest opinion on the product.

In one example, the investigator gave the product – a smartwatch – a two-star review. “They were told by the seller to rewrite it because the product was free, so it “is the default to give five-star evaluation”, said Which?

In another, the investigator was told that a “refund will be done after a good five-star review with some photo” after receiving a pair of wireless headphones. But after posting a three-star review with photos they were told they would not be refunded unless they wrote a five-star review. The investigator refused, so did not get refunded for the purchase.

When the Guardian searched the Amazon UK Reviewers Facebook group – which has more than 25,000 members – it found postings appearing almost every couple of minutes from companies around the world offering to pay for positive reviews. For example, on Friday, one company was seeking “UK reviewers only” for a “4k action camera waits for review Refund via Paypal just send me your amazon profile”.

Postings on the Amazon UK Reviewers Facebook group on 19 October. Photograph: Facebook

The postings reveal the globalised nature of the review factories, with what appear to be mostly far-eastern manufacturers looking for consumers in western markets to access Amazon and post the reviews.

Bogus online reviews have plagued the internet for years, yet Which? said they remained highly influential. As part of its investigation, it polled Which? members and found that 97% use online reviews when researching a product – but three in 10 (31%) were disappointed after buying a product because of excellent feedback scores.

Alex Neill, the Which? managing director of home products and services, said: “Sellers are effectively ripping people off with paid-for reviews. They don’t represent an honest and impartial opinion but instead mislead people into buying products that they might have otherwise avoided.”

Which? shared its findings with Amazon and Facebook.

In its response, Amazon said: “We do not permit reviews in exchange for compensation of any kind, including payment. Customers and marketplace sellers must follow our review guidelines and those that don’t will be subject to action including potential termination of their account.”

It added that a “small fraction of reviews” violated its guidelines, that it had closed abusive accounts and in some cases had taken legal action.

Facebook said: “Facilitating or encouraging the trade of fake user reviews is not permitted on Facebook. We urge people to use our reporting tools to flag content they suspect may violate our standards so that we can take swift action.”

In September, a man was jailed in Italy for selling fake Trip Advisor reviews in a landmark court ruling. He had offered his services to hundreds of hotels and restaurants across Italy, but a court found that writing fake reviews under a false identity was a crime under Italian law.