"Furthermore, the government of Japan has been aware of this very real health risk for more than 20 years," it says.
The study, "Mercury Rising", is written by the Environmental Investigation Agency (EIA), a small group that specialises in using undercover techniques to expose environmental abuse.
Its latest research is timed to coincide with a meeting, running from June 16 to 19 in Berlin of the International Whaling Commission (IWC), where Japan is expected to relaunch its attempt to end a 1986 global moratorium on commercial whaling.
Japan has sidestepped the IWC ban by hunting four species of large whale - North Pacific minkes, Bryde's, sei and sperm whales - supposedly for scientific research.
It also allows a catch of more than 22,000 smaller cetaceans, from nine dolphin, porpoise and small whale species, each year.