The UK regulator is investigating both companies over possible breach of consumer protection law.
The announcement follows a series of Which? investigations into fake reviews on Amazon, Google and other platforms, and the persistent problems they cause for consumers.
Amazon and Google will be formally investigated by the Competition and Markets Authority (CMA) over concerns that not enough is being done to combat fake reviews.
The CMA will open a formal probe intended to gather further information and to determine whether the two sites may have broken consumer law, following an initial investigation in May 2020 to assess the internal systems of several platforms.