The competition watchdog has opened an inquiry into whether Amazon and Google are making enough effort in tackling fake reviews on their platforms.
The Competition and Markets Authority (CMA) said its initial work on the issue, started just over a year ago, raised a number of concerns and it would now "gather further information to determine whether these two firms may have broken consumer law by taking insufficient action to protect shoppers".
The issue is more complicated than it would appear on the surface as it takes in the activities of paid-for review manipulation operators - a bugbear of consumer groups.
Image: Google's own review system is facing scrutiny
Which? issued a report in February that found fake reviews for goods sold on Amazon's Marketplace were being touted in bulk for as little as £5 each to third-party sellers.
It also claimed to have uncovered 10 sites also soliciting positive reviews in exchange for free products or cash.
The CMA said its formal inquiry would examine a number of areas including "suspicious" patterns of behaviour where "the same users have reviewed the same range of products or businesses at similar times to each other and there is no connection between those products or businesses".
That work, the regulator added, would also take in where the review suggests that the reviewer has received a payment or other incentive to write a positive review.
The CMA said it was yet to take a view on whether the companies had broken consumer protection law but has the power to issue formal commitments, if warranted, rising to court action if any remedies it demands are ignored.
It builds on action taken by the CMA last year over the trading of fake reviews which resulted in Facebook, Instagram and eBay removing groups and banning individuals for buying and selling fake reviews on their sites.
Its chief executive, Andrea Coscelli, said of its latest inquiry: "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 5-star reviews to give their products or services the most prominence, while law-abiding businesses lose out.
"We are investigating concerns that Amazon and Google have not been doing enough to prevent or remove fake reviews to protect customers and honest businesses. 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."