U.S. tech firms are in more hot water in Britain.
The country's competition regulator started a formal investigation on Friday (June 25).
It's into whether Amazon and Google have done enough to prevent or remove fake reviews.
The Competition and Markets Authority said it would gather more information to decide if the firms broke consumer law.
The regulator said it was also concerned that Amazon’s systems had failed to prevent and deter some sellers from manipulating product listings.
Last year action taken by the CMA over the trading of fake reviews resulted in Facebook, Instagram and eBay removing groups and banning individuals.
The CMA said it has not reached a view on whether Amazon and Alphabet's Google have broken the law.
Amazon said it would continue to assist with enquiries.
Google did not immediately respond to a request for comment.