Competition officials are to examine whether people and businesses have been able to post fake reviews online with too much impunity.
The Competition and Markets Authority has opened a formal investigation into whether Amazon and Google have done enough to crack down on the practice.
CMA 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 watchdog said that over the past year it has become concerned that the two technology giants 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.
“It’s important that these tech platforms take responsibility and we stand ready to take action if we find that they are not doing enough,” Mr Coscelli said.
The CMA said its concerns have been prompted by a year-long initial inquiry, which caused it to launch this 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.