Amazon’s customer review system is being undermined by a flood of “fake” five-star reviews for products from unfamiliar brands, a new investigation claims.

The consumer group Which? analysed the listings of hundreds of popular tech products in 14 online categories including headphones, dashcams, fitness trackers and smartwatches, checking for telltale signs of suspicious reviews.

Its researchers found that top-rated items were dominated by brands with names such as Itshiny, Vogek and Aitalk, which in many cases had thousands of unverified reviews – meaning there was no evidence that the reviewer had even bought or used the item.

Many items also boasted a high number of five-star ratings posted in a short space of time – another indicator suggesting inauthentic reviews.

With headphones, all the products on the first page of results sorted by average customer review were from little-known brands and 87% of more than 12,000 reviews for these products were by unverified purchasers.

Seventy-one per cent of the headphones had perfect five-star ratings, while some included reviews for unrelated products such as soap dispensers. One set of headphones made by the brand Celebrat had 439 reviews. All were five-star, all unverified, and all arrived on the same day.

Which? found similar results when searching for smartwatches, with unverified reviews making up 99% of reviews for the top four products.

“Our research suggests that Amazon is losing the battle against fake reviews, with shoppers bombarded by comments aimed at artificially boosting products from unknown brands,” said Natalie Hitchins, the head of home products at Which?.

“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.”

Neither Which? nor the Guardian were able to contact any of the brands cited in the report, or to identify the source of the suspicious reviews.

Amazon said in a statement: “[We] invest significant resources to protect the integrity of reviews in our store because we know customers value the insights and experiences shared by fellow shoppers. 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.”

A Guardian analysis also recently found that some items on Amazon are bundled together when they share a title, even if they are a different translation of a book or a remake of a film, making it difficult for readers to know which version they are buying.