The International Court of Justice has ruled that Japan's Antarctic whaling programme is not for scientific purposes and has ordered a temporary halt.
Australia had sued Japan at the UN dispute resolution court in the hope of ending whaling in the Southern Ocean.
Reading a judgment by the court's 16-judge panel, presiding judge Peter Tomka, of Slovakia, said Japan has not justified the large number of minke whales it takes under its programme, while failing to meet much smaller targets for fin and humpback whales.
The court ordered a halt to the issuing of whaling permits until the programme has been revamped.
Judge Tomka said: " The evidence does not establish that the programme's design and implementation are reasonable in relation to achieving its stated objectives."
He noted among other factors that Japan had not considered a smaller programme or non-lethal methods to study whale populations, and that it cited only two peer-reviewed scientific papers relating to its programme from 2005 to the present - a period in which it has harpooned 3,600 minke whales, a handful of fin whales, and no humpback whales.
The decision is a major victory for Australia and environmental groups that oppose whaling on ethical grounds, though it will not mean the end of whaling.
Japan has a second, smaller programme in the northern Pacific, while Norway and Iceland reject a 1986 moratorium on commercial whaling imposed by the International Whaling Commission.
Japan had argued that Australia's challenge was an attempt to force its cultural norms on Japan, equivalent to Hindus demanding an international ban on killing cows.
Though consumption of whale meat has declined in popularity in Japan in recent years, it is still considered a delicacy by some.
Japan has pledged to abide by the court's ruling.