Britain's competition regulator started a formal investigation on Friday into whether Amazon and Google may not have done enough to prevent or remove fake reviews.
Along with regulators in the United Sates and the European Union, the Competition and Markets Authority (CMA) has stepped up its scrutiny of big tech firms in recent years.
The British regulator said it will gather more information to decide if the firms may have broken consumer law by taking insufficient action to protect shoppers. Both Google and Amazon said they were continuing to assist the CMA.
CMA action last year over the trading of fake reviews resulted in Facebook, Instagram and eBay removing groups and banning individuals for buying and selling fake reviews on their sites.
Google said on Thursday it would delay blocking tracking cookies on its Chrome browser following intervention by the CMA.
The CMA started its investigation into reviews in May 2020, focusing on the internal systems and processes of several platforms for identifying and dealing with fake reviews.
The regulator said it was also concerned that Amazon's systems had failed adequately to prevent and deter some sellers from manipulating product listings, through for example co-opting positive reviews from other products.
"Our worry is that millions of online shoppers could be misled by reading fake reviews and then spending their money based on those recommendations," Reuters quoted CMA's Chief Executive Andrea Coscelli as saying in a statement.
"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."
