Scientists say they have found clear proof that meat from whales captured under Japan's whaling programme is being sold in US and Korean eateries.
The researchers say they used genetic fingerprinting to identify meat taken from a Los Angeles restaurant as coming from a sei whale sold in Japan.
They say the discovery proves that an illegal trade in protected species still exists.
Whale meat was also allegedly found at an unnamed Seoul sushi restaurant.
Commercial whaling has been frozen by an international moratorium since 1986.
But a controversial exemption allows Japan to kill several hundred whales each year for what is termed scientific research.
The meat from these whales is then sold to the public in shops and restaurants in that country.
A team of scientists, film-makers and environmental advocates say they collected samples of whale meat being sold in sushi restaurants in both the US and South Korea late last year.
A genetic analysis of meat found in Los Angeles showed that it was identical to meat from a sei whale being sold in Japan in 2007. This species is said by environmentalists to be in danger of extinction.
Criminal proceedings have started against the Los Angeles restaurant caught selling the whale meat.
It has now closed but its chef and owners face heavy penalties.
Writing in the Royal Society journal, Biology Letters, the researchers involved say that trading in this meat is banned between countries that have signed the Convention on International Trade in Endangered Species.
The researchers also visited an unnamed restaurant in the South Korean capital Seoul where they say they purchased 13 whale products on two occasions in June and September 2009.
Four came from an Antarctic minke whale, four from a sei whale, three from a North Pacific minke, one from a fin whale and one was from a Risso's dolphin, the researchers say.
The DNA profile of the fin whale meat genetically matched meat that had been bought in Japanese markets in 2007, they report.
They argue that Japan should be required to make public a register of the DNA of all the whales it catches so that illegally traded meat can be tracked.