The Competition and Markets Authority announced that it has opened a formal probe into whether internet giants Amazon and Google have done enough to crack down on the practice.
The watchdog’s chief executive, Andrea Coscelli, said: “Our worry is that millions of online shoppers could be misled by reading fake reviews and then spending their money based on those recommendations.
“Equally, it’s simply not fair if some businesses can fake five-star reviews to give their products or services the most prominence, while law-abiding businesses lose out.”
The CMA said that over the past year it has become concerned that the two tech firms are not doing enough to detect fake and misleading reviews or suspicious behaviour.
In some cases users might have reviewed the same range of products or businesses, or at times reviews suggest that the writer was paid or given another incentive to write the post.
It questioned whether the two are doing enough to investigate and promptly remove fake and misleading reviews from their platforms, and impose adequate sanctions on reviewers or businesses engaged in the practice.
The CMA said its concerns have been prompted by a year-long initial inquiry, which caused it to launch a formal investigation.
If it finds that the two companies are not doing enough, the CMA could force them to change how they work.
But officials stressed that they have not yet reached a view on whether either has broken the law.
Last year Facebook, Instagram and eBay removed groups and banned individuals for buying or selling fake reviews on their sites.
A spokesperson for Amazon said: “To help earn the trust of customers, we devote significant resources to preventing fake or incentivised reviews from appearing in our store.
“We work hard to ensure that reviews accurately reflect the experience that customers have had with a product.
“We will continue to assist the CMA with its inquiries and we note its confirmation that no findings have been made against our business.”