An Australian court has ruled that the activities of the Japanese whaling fleet are illegal and ordered it to halt its operations.
Japan does not recognise Australia's Antarctic whale sanctuary
The Federal Court said Japan had broken domestic law by killing whales in Australian waters off Antarctica.
It is not clear what effect the ruling will have, as Japan does not recognise Australia's Antarctic claim.
But it is likely to raise tensions between the two nations, which are far apart on the issue of whaling.
The Japanese fleet plans to kill about 900 minke whales and 50 fin whales by mid-April as part of what it describes as a scientific research programme.
But Australia and other nations say the research goals could be achieved using non-lethal methods and call the programme a front for commercial whaling.
The case, initially brought in 2004 by the Humane Society against ship operator Kyodo Senpaku Kaisha, was delayed for several years because of fears it could cause a diplomatic row.
At the court in Sydney, Justice James Allsop ruled that the fleet had broken Australia's Environment Protection Act and issued an injunction ordering it to stop whaling.
"The court orders that the respondent be restrained from killing, injuring, taking or interfering with any Antarctic minke whale, fin whale or humpback whale in the Australian whale sanctuary," he said in his ruling.
The judgement, however, is unlikely to end the whaling. Japan does not recognise Australia's Antarctic claim or its declaration of a whale sanctuary in the area.
Tokyo has said in the past that it will ignore any injunction resulting from the case.
The court also has no jurisdiction outside Australia and Justice Allsop said it was unlikely that his ruling could be enforced.
Australia's Environment Minister Peter Garrett said that he was still considering the decision, but emphasised that surveillance operations were in place.
An Australian patrol ship and an aircraft have already been sent south to assemble photographic and video material of the fleet for a possible legal challenge in international courts.
Ships from environmental group Greenpeace and the more radical Sea Shepherd are also trailing the fleet in a bid to disrupt the hunt.
They say they have forced the main factory ship, the Nisshin Maru, out of the whaling zone, temporarily preventing the fleet from hunting.
Japanese whaling officials have condemned the two groups, blaming them for past collisions and describing their activities as illegal harassment.