Fake reviews for products sold on Amazon's Marketplace are being sold online "in bulk", according to Which?
The consumer group found 10 websites selling fake reviews from £5 each and incentivising positive reviews in exchange for payment or free products.
It suggested the firm was facing an "uphill struggle" against a "widespread fake reviews industry".
An Amazon spokesman said: "We remove fake reviews and take action against anyone involved in abuse."
The retail giant's Marketplace allows other retailers to sell their goods via the Amazon website.
Which? identified websites offering review services for goods for sale on Amazon Marketplace that violated the firm's terms and conditions.
These included "packages" of fake reviews available for sellers to buy for about £15 individually, as well as bulk packages starting at £620 for 50 reviews and going up to £8,000 for 1,000.
The group also suggested that five of the businesses it looked at had more than 702,000 "product reviewers" on their books.
Product reviewers are offered small payments ranging from a few pounds up to more than £10, alongside free or discounted products. They can even take part in "loyalty schemes" and earn themselves premium goods, from children's toys to exercise equipment.
The websites Which? reviewed also offered advice on how to write reviews so as not to arouse Amazon's suspicion, and in some cases had criteria for reviewers to meet to qualify for rewards, it said. They included leaving reviews that were at least two sentences long, or including photos, for example.
How to spot fake online reviews:
- Be sceptical. This is probably your best weapon, especially if there is an unusually high number of reviews of one product compared with others in that category.
- Read and read again. Look for the same language used in multiple reviews.
- Brands you don't know. If you don't recognise the brand, check to see if it has its own legitimate-looking website, with clear contact details.
- Be wary of products with lots of pictures or videos. Sellers can incentivise fake reviewers to encourage people to add photos and videos.
Natalie Hitchins, head of home products and services at Which?, called on the Competition and Markets Authority (CMA) to look into the sale of fake reviews urgently.
"The regulator must crack down on bad actors and hold sites to account if they fail to keep their users safe," she said.
"If it is unable to do so, the government must urgently strengthen online consumer protections."
A previous investigation by the consumer group found dozens of Facebook groups with sellers offering refunds or commissions in exchange for fake, favourable reviews.
It led to Facebook and eBay signing agreements with the CMA to "better identify, investigate and respond to fake and misleading reviews".
But Ms Hitchins said: "Amazon, and other online platforms, must do more to proactively prevent fake reviews infiltrating their sites so that consumers can trust the integrity of their reviews."
An Amazon spokesman pointed out that it worked with other tech firms to "report bad actors", but added that online retailers "cannot do this alone".
He said that customers need to be able to trust the reviews they see online and that more enforcement powers should be given to regulators such as the CMA.
As demand for online shopping grew at the beginning of the coronavirus pandemic, the UK regulator launched a review into how different websites detect and respond to fake and misleading reviews.
The authority warned last May it would "not hesitate" to take action if sites were disobeying the law.
That could include taking major retailers to court, although it did not name the specific websites it would be investigating.