The Environmental Investigation Agency in London has warned that false labelling of the meat means that children will also be consuming mercury-laden dolphin and porpoise meat.
"We buy samples of whale meat every year in Japan to have the DNA analysed," the agency said. "The evidence is that around 10 per cent of the meat labelled as whale is in fact other small cetaceans such as porpoise."
The agency said that this meat contained 10 times the amounnt of mercury allowed by law in Japan.
Japan also hunts thousands of dolphins and porpoises off its coast. The meat is sold commercially and ends up in restaurants, schools and even hospitals.
Tokyo disputes the agency's findings and says it is the country's cultural right to eat whales and dolphins.
Children widely enjoyed eating whale - a word used generically in Japan to include all cetaceans - for school lunch until 1982, when Japan was forced to stop commercial whaling. "We want school children to know Japan's traditional dietary culture through whale meat, which was popular in the past," an official of an education board told Kyodo News.