A UK regulator said Friday it has opened formal cases against Amazon and Google over concerns that the US tech giants are failing to shield consumers from fake reviews.

The Competition and Markets Authority (CMA) said that it had concerns the companies were not doing enough to protect customers from fake reviews, in "possible breaches of consumer protection law".

The problem centres on reviews that customers can post online after buying items, with concerns that some businesses are posting fake reviews or paying people to write them -- distorting the star ratings that many consumers rely on.

"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," said the CMA's chief executive, Andrea Coscelli.

The CMA is taking further action after it began a probe last year into how these platforms detect fake reviews, deal with and punish those who post them.

If it finds that Amazon and Google have broken the law, it can order them to formally commit to change, under threat of court action.