Amazon's system of customer reviews and ratings is being undermined by a flood of "fake" five-star reviews for products made by unknown brands, a consumer group has claimed.
Which? investigated hundreds of technology products in 14 popular categories, including headphones, dash cams, fitness trackers and smart watches, checking for tell-tale signs of suspicious reviews.
It found that the top-rated items were dominated by unknown brands, which in many cases had thousands of unverified reviews - meaning there is no evidence that the reviewer has even bought or used the product.
Many also had a "suspiciously high" number of five-star ratings that had been loaded on to Amazon's review pages in a short space of time, suggesting they were fake, the watchdog said.
When Which? searched for headphones, all the products on the first page of results were from unknown brands and almost nine in 10 (87%) of more than 12,000 reviews for these products were from unverified purchasers.
Seven out of 10 (71%) of the headphones in the results had five-star customer ratings, while some included reviews for unrelated products such as soap dispensers.
One set of headphones by an unknown brand had 439 reviews. All were five-star, all unverified, and all arrived on the same day, Which? reported.
Review experts ReviewMeta, which analysed the Which? Findings, said it believed "pretty much every five-star unverified review" of the top 10 pairs of headphones was "fake", with a spokesman adding: "I'm shocked we've been seeing this so much on Amazon - it seems so obvious and easy to prevent."
Analysis of smart watches again found every device on the first page of results when sorted by average customer review was from an unknown brand, with unverified reviews making up 99% of reviews for the top four products.
The pattern was repeated with action cameras, fitness trackers and wireless security cameras, with more than nine in 10 of the top-rated products in each category made by unknown brands and boosted by a deluge of "dubious" reviews.
Which? said it had been unable to establish the source of the unverified reviews.
The Competition and Markets Authority estimates that £23 billion a year of UK consumer spending is potentially influenced by online reviews.
Natalie Hitchins, Which? head of home products and services, said: "Our research suggests that Amazon is losing the battle against fake reviews - with shoppers bombarded by dubious comments aimed at artificially boosting products from unknown brands.
"Amazon must do more to purge its websites of unreliable and fake reviews if it is to maintain the trust of its millions of customers.
"To avoid being misled and possibly buying a dud product, customers should always take reviews with a pinch of salt and look to independent and trustworthy sources when researching a purchase."
Amazon said it used a combination of teams of investigators and automated technology to prevent and detect inauthentic reviews at scale.
A spokesman said: "Amazon invests significant resources to protect the integrity of reviews in our store because we know customers value the insights and experiences shared by fellow shoppers. Even one inauthentic review is one too many.
"We have clear participation guidelines for both reviewers and selling partners and we suspend, ban, and take legal action on those who violate our policies."