Amazon sellers are bombarding customers with incentives in exchange for five-star reviews, according to Which? research.

The research revealed that one in 10 Amazon shoppers have been offered bribes such as gift cards, refunds and free products after purchasing their products.

The consumer champion said it is concerned that unscrupulous sellers are cheating their way to the top of the Amazon rankings by offering customers rewards.

Which? surveyed 1,556 adults in July, and found that in the last year 10% of UK customers had received a card in their packaging offering them an incentive for leaving a five-star review.

4% were offered a reward for changing a negative review to a positive one.

A further 8% highlighted that they were approached by a seller via emails or other communication method, they estimate some 4.5 million customers have potentially been targeted.

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Sellers on Amazon have gone as far giving a full refund to shoppers who have left five-star reviews.

One customer was sent £50 in Amazon vouchers plus a full refund for leaving a positive review.

Another said they were incentivised to change their negative review to a positive one.

Which? said leaving a positive review in exchange for payment causes harm to other consumers.

Previous Which? research highlighted that customers are more than twice as likely to choose poor-quality products as a result of dishonest reviews.

Rocio Concha, Which? director of policy and advocacy, said:“It’s clear that sellers on Amazon are bombarding customers with incentives in order to cheat the system and we have seen evidence that they are successfully evading Amazon’s defences.

“Amazon and other review-hosting websites need to step up and do more to banish fake reviews from their platforms by taking measures that ensure the reviews they host are genuine.

“The government must make hosting fake reviews a criminal offence in its Digital Markets, Competition and Consumers Bill so that regulators can take strong action to crack down on the problem.”

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On one occasion, a customer reported that she had bought a handheld Lyyxll vacuum from Amazon. The vacuum had 4,264 ratings, with 55% being five-star reviews and had sold more than 1,000 in just one month.

The customer said she found a letter in the box offering a £10 gift card in exchange for a five-star review.

An internal investigation at Which? found that the vacuum received multiple types of review manipulations and worryingly some people reported dangerous faults with the product.