Unscrupulous sellers are beating Amazon’s security systems with tactics such as bribery and hacking to post fake reviews for their products, consumer group Which? has claimed.

The watchdog said features designed to make the online giant’s website more user-friendly were being “abused on a grand scale” to mislead customers.

It scrutinised thousands of listings and reviewed almost 90 reports of fake reviews in recent weeks, including sellers offering buyers bribes of cash and gift vouchers in return for dishonestly boosting a review to five stars.

Some sellers are also hacking genuine Amazon accounts to post fake reviews, Which? said.

In one case, a single account was used to post thousands of fake reviews, while some victims had been told they must manually remove fake reviews posted in their name.

Fraudulent sellers were also exploiting an Amazon feature called “product variation”, which is used by legitimate sellers to group reviews for the same product in one place when it is available in different sizes and colours.

However, Which? said some sellers had created false variations, allowing them to artificially multiply the number of positive reviews attributed to a product while evading detection.

Which? found a pair of SDFLAYER headphones with 40 different meaningless colour variations that enabled the seller to coordinate a steady stream of fake five-star reviews without Amazon’s systems flagging any suspicious activity on the product.

The product topped the headphones search listings at the start of June 2019.

Another feature called “product merging”, used by legitimate sellers to bring together reviews of similar items under one listing, is also being used by some to transfer positive reviews of old products to boost the ratings of new or existing listings.

In one case, Which? found reviews for a soap dispenser and a phone screen cover listed under a pair of headphones to artificially boost the product’s rating.

Last year, a separate Which? investigation revealed ‘review factory’ groups on Facebook with tens of thousands of members generating incentivised positive ratings for products listed on Amazon.

Despite reporting the issue to Facebook, Which? found review groups that were still “highly active” as recently as last month, with one posting 133 times in an hour.

The Competition and Markets Authority (CMA) has announced that Facebook and eBay must tackle the sale of fake reviews through their sites.

Which? head of home products and services, Natalie Hitchins, said: “Writing or commissioning fake or incentivised reviews is in breach of consumer law and can lead to criminal action against the individuals responsible.

“It is unacceptable that consumers continue to be misled into buying poor quality or even unsafe products by the current system of reviews and rankings.

“Online platforms must do more to tackle fake reviews, going above and beyond the current approach.

“If they fail to put more rigorous systems in place, then the CMA must intervene to ensure that fake reviews and other misleading tactics can be stamped out.”

An Amazon spokesman said: “Any attempt to manipulate customer reviews is strictly prohibited at Amazon.

“We use a combination of teams of investigators and automated technology to prevent and detect inauthentic reviews at scale, and to take action against the bad actors behind the abuse.”

Amazon said the best way for customers to ensure they did not respond to a false or phishing e-mail was to always go directly to their account on Amazon to review or make any changes to orders or the account.

A Facebook spokesman said: “Fraudulent activity is not allowed on Facebook, including the trading of fake reviews, and we have removed all of the groups Which? reported to us.

“We know there is more to do to tackle this issue, which is why we’ve tripled the size of our safety and security team to 30,000 and continue to invest in technology to help proactively prevent this kind of abuse."

Josie Clarke is PA Consumer Affairs Correspondent.