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.
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.”
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.
It is also revealed that there were more than 100 reviews for a completely different product, 90 of the reviews were five stars, and the rest were three or four.
This is known as ‘review merging’ where sellers merge the positive reviews of a dormant Amazon listing with their own to boost the number of five-star reviews.
The consumer group says Amazon has access to a huge amount of data to tackle this problem and has called on platforms to do more to ensure reviews are genuine.
Amazon said it has clear policies that prohibit reviews abuse which sellers must adhere to, but these are often coordinated outside the website, making it more difficult to detect.
The American online shopping website said: “There is absolutely no place for this kind of activity and we suspend, ban, and take legal action against sellers who violate our policies.
“We had already taken action against the sellers identified by Which? prior to publication of the report and removed a number of reviews.
“However, we are constantly developing new ways to prevent this and we urge any customer who has been contacted by a seller about a review to report it so we can take action.
“We are committed to ensuring our reviews remain trustworthy and our goal is to ensure that every review in Amazon’s stores reflects customers’ actual experiences.”
A consultation between the consumer champion and the government is currently underway to help tackle this issue.
In response to the growing number of misleading reviews, Which? released a new report to ensure online platforms like Amazon, Facebook, Google and Trustpilot display honest reviews.