Amazon customers are being deceived by reviewers giving favourable ratings on products offered to them free or at a big discount, an investigation by The Timeshas found.

Last year the online retailer was forced to admit that some reviewers were posting fictitious five-star reviews in exchange for money. It has taken legal action against the cheats.

In the new scam, products are offered at a discount on an external site. Shoppers are issued with a voucher code which they redeem on Amazon and purchase at a lower price before providing an “honest and fair review”.

The writers are meant to include a disclaimer of this, but it is often buried at the end of long posts or hidden behind the “read more” button. The sites insist that the appraisals do not have to be positive, but administrators select which individuals receive the discount coupon.

The Times has found that Amazon inadvertently endorses the “biased” opinions by marking them as a “verified purchase”; a contradiction of the company’s policy. It states that this should be the case only if the reviewer “didn’t receive the product at a deep discount”.

The reviews are contributing to the overall rating that buyers see on screen, meaning that four-star and five-star items could have had their scores artificially bumped up. Consumer groups say the reviews have the “potential to destroy the trust of customers”.

Some of the “impartial” reviewers have rated hundreds of items in a short time. One has rated 738 items since the start of September, producing 60 in a single day. Most products were given five stars. Another user has posted 300 reviews since October, some identical. A set of hair curlers worth £74.99 — for which the user paid less than half price — earned high praise: “I honestly love this curler; it is hands down, the best product out there on the market today. I received this product at a discount, in exchange for an honest review.”

All but three of the 32 five-star reviewers for the product received a discount.

The Times found examples of “honest and fair” reviews across Amazon’s departments, ranging from kitchen appliances and electronics to books. “Impartial” ratings for a £70 camera, discounted to £40 on the external site, spoke of “excellent customer service”. While users are told to be impartial, one invitation to review said users “must” comment on specific features.

Only low-value products, from little-known manufacturers, appear to be offering discount prices for good reviews at present, but their high ratings could push them into the Amazon searches of online shoppers — tricking them into a purchase.

James Walker, of Resolver, a consumer watchdog, said Amazon “needs to go further” to tackle dubious write-ups. “Biased or fake reviews are a real and clearly increasing problem,” he said.

A spokesman for Amazon UK said: “Our goal is to make reviews as useful as possible for customers. We use a number of mechanisms to detect and remove the small fraction of reviews that violate our guidelines, and we terminate accounts. We’ve recently filed lawsuits against a number of individuals and businesses who were abusing the system.”