The CMA said it plans to look into whether the firms broke consumer law by not doing enough to protect shoppers.
The formal probe follows an initial CMA investigation, which opened in May last year and looked at how consumer sites deal with fake reviews.
The CMA said its work over the past year has identified concerns over whether the two firms are doing enough to detect fake reviews and misleading behaviour, investigate and remove the reviews, and hit the review writers with adequate sanctions.
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.
"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."