A number of groups on Facebook, some with tens of thousands of members, offer free or discounted products in exchange for 5* reviews.

An investigator at campaign group Which? set up Amazon and Facebook accounts and joined several ‘reward for reviews’ groups.

They were told to order a specific item through Amazon and once they had written a review and shared a link, they would receive a refund for the cost of the item.

But where the review was not positive enough, no refund was received and the seller could not be contacted.

In one case, the investigator gave a two-star review for a smartwatch and they were told by the seller that the review would need to be re-written because they had received it for free so should by default receive a 5* rating.

A Which? survey of 2,073 people showed that 97% of people use online reviews when researching a purchase, with 31% being disappointed after buying a product because of excellent feedback scores.

The Competition and Markets Authority (CMA) estimates that £23bn a year of UK consumer spending is influenced by online customer reviews.

Alex Neill, 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.

“We all like to do research before buying something. Watch out for unscrupulous sellers and use independent review sites, such as Which?, to make sure you’re getting the products you want.”

Amazon and Facebook response

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

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.”