Judges at the highest U.N. court have ordered Japan to halt whaling in the Antarctic, rejecting the country's long-held argument that the catch was for scientific purposes and not primarily for human consumption.
Tokyo said it was disapointed but would abide by the decision, while activists said they hoped it would bring closer a complete end to whaling around the world.
The International Court of Justice sided with plaintiff Australia in finding that the scientific output of the whaling programme did not justify the number of whales killed.
The tribunal said no further licences should be issued for scientific whaling, where animals are first examined for research purposes before the meat is sold to consumers.
"In light of the fact the JARPA II (research programme) has been going on since 2005, and has involved the killing of about 3,600 minke whales, the scientific output to date appears limited," Presiding Judge Peter Tomka of Slovakia said.
Japan signed a 1986 moratorium on whaling, but has continued to hunt up to 850 minke whales in the icy waters of the Southern Ocean, as well as smaller numbers of fin and humpback whales, citing a 1946 treaty that permits killing the giant mammals for research.
Japan was "deeply disappointed" by the ruling, but it would comply, said Koji Tsuruoka, the country's chief lawyer before the court. He said the government would need to study the ruling before taking any further action.
Judges agreed with Australia that the research - two peer reviewed papers since 2005, based on results obtained from just nine killed whales - was not proportionate to the number of animals killed.
The judgment is an embarrassment to Japan, but Tokyo could continue whaling if it devised a new, more persuasive programme of scientific that required "lethal catch" of whales, or if it withdrew from the whaling moratorium or the 1946 International Convention for the Regulation of Whaling.
Whaling was once widespread around the world, but Japan is now one of only three countries, alongside Iceland and Norway, that continue the practice. The meat is popular with Japanese consumers who consider it a delicacy.
Norway, the other main whaling nation, in 1993 shifted away from scientific whaling to "commercial" catches, where the meat is sold directly to consumers.
Norway set a quota of 1,286 minke whales in the north Atlantic in last year's summer hunt, saying stocks are plentiful in the region. Fishermen rarely catch the full quota, partly because demand has sunk in recent years.
Iceland and Norway do not claim to be carrying out research, openly hunting whale meat for commercial purposes, meaning the ICJ's ruling has no immediate consequences for them. But activists said the ruling reflected a gradually changing climate that would put an end to whaling.
"Whaling is under immense scrutiny from the international community, and the pincer movement on these countries is ever tightening," said Claire Bass, wildlife campaigner at the World Society for the Protection of Animals.