Japan's whaling fleet is believed to be quitting the Antarctic under heavy conservationist and diplomatic pressure, just half way through its worst ever season.

The Japanese Government has decided to cut short the season and the fleet is heading back to port, sources in Tokyo told the International Fund for Animal Welfare.

The sudden departure has raised hopes Japan may be moving to end its widely opposed, 23-year-old "scientific research" program, which has killed around 10,000 Antarctic whales.

"Under pressure from all fronts, the Japanese whaling fleet is apparently withdrawing early this season from the internationally recognised sanctuary around Antarctica," said IFAW's global whales campaign director, Patrick Ramage.

"We hope this is a first sign of Japanese government decision makers recognizing there is no future for whaling in the 21st Century and that responsible whale watching, the only genuinely sustainable use of whales, is now the best way forward for a great nation like Japan," Mr Ramage said.

No immediate official confirmation was available from Japan. But the factory ship, Nisshin Maru, was today steaming towards Drake Passage, below South America, pursued by the Sea Shepherd group's vessel Bob Barker, having left its nominated whaling grounds 2,000 nautical miles behind.

Ahead of the Nisshin Maru, the Chilean government said it planned to use naval assets to closely monitor the approaching factory ship.

Chile has permanently banned whaling in its waters, and also forbids the transport of cetacean parts through them, but Nisshin Maru should be able to navigate Drake Passage without entering the Chilean exclusive economic zone.

The whereabouts of the fleet's three harpoon ships is unknown, but they have been unable to kill whales without the Nisshin Maru to process them.

A smaller whaling fleet came under sustained Sea Shepherd pressure from the delayed outset of its season this year, sharply reducing its capacity to catch a quota of up to 935 minke whales and 50 fins.

The activists located the fleet as soon as it reached the Antarctic, kept two of the three Japanese harpoon ships engaged for weeks, fouled the propellors of one, delayed a fleet refuelling operation, and then sent Nisshin Maru on the run.

The whaling industry is also suffering at home in Japan, with revenue falling as whale meat sales plummet, and with key figures in the Fisheries Agency of Japan forced to apologise for running a black market in the meat.

Following the failure of International Whaling Commission peace talks, the main diplomatic game has shifted to Australia's International Court of Justice case against Japan, which is due to be laid out in full in May.

A joint statement by Prime Minister Julia Gillard and New Zealand Prime Minister John Key today reaffirmed their commitment to the case, and to the elimination of whaling in the Southern Ocean.

Latin American members of the International Whaling Commission this week issued another joint statement urging Japan to stop scientific whaling in Antarctic waters and respect sanctuaries for the species.