Sea Shepherd captain Paul Watson expects the Southern Ocean whale-hunting season to be cut short after a dramatic clash prevented Japanese boats refuelling.
The whaling fleet abandoned an attempt to refuel from a tanker on Wednesday when Sea Shepherd vessels intervened.
The conservation activist group says three of its boats were then rammed by the Japanese ship Nisshin Maru.
Sea Shepherd says the whaling vessels were acting illegally by refuelling in an area below 60 degrees south, an activity prohibited by an Antarctic treaty.
“The ICR (Japanese Institute for Cetacean Research) has announced a temporary halt in their whaling operations, which over the last few years usually means they're ending it for the season,“ Captain Watson said from the Sea Shepherd ship Steve Irwin.
But Japan has vowed to continue its whale hunt.
“We are keeping our whaling programme,” an official at Japan's Fisheries Agency tsaid, denying claims that Japan was forced to suspend its whale hunt after collisions with boats crewed by anti-whaling campaigners.
The official also repeated Tokyo's claim that the conservationists had rammed Japanese whaling ship the Nisshin Maru on Wednesday, their worst confrontation in the Southern Ocean in three years.
On Wednesday, the conservationist group - which earlier this month lost a battle at the US Supreme Court over an order to steer clear of Japan's whaling fleet - accused the Japanese side of deliberately colliding with its vessels.
A spokesman for Japan's Institute of Cetacean Research said on Thursday that its ship could not be refuelled “due to Sea Shepherd's dangerous activities”.
Captain Watson said he expected the Korean tanker Sun Laurel to refuel the Japanese vessels around 300 nautical miles further north.
But, with 18 days left in the whaling season, he said it was unlikely the fleet would then head south again.
“I feel that this is the end of it,” he said.
Captain Watson rejected a Japanese suggestion his vessels had provoked the dangerous confrontation on Wednesday.
“Their argument is that we hit their fist with our face,” he said. “It was more like a case of road rage.”
He said court action was unlikely after attempts to seek compensation for the sinking of the Ady Gil in 2010 had failed.
Sea Shepherd estimates only 12 whales were caught this season, which put in doubt the future of the annual hunt.
Captain Watson said, if it continued, Australia should send a vessel to keep the peace, even if it was successful in having whaling outlawed in action being taken at the International Court of Justice.
“I think the Japanese will just ignore the court,” Captain Watson said.
“Then I would hope that Australia would send a vessel down to enforce that ruling.”
The Federal Government has ruled out sending a navy ship to Antarctic waters.
“It's not something that we try to settle in a carpark, we settle it in a court,” Environment Minister Tony Burke told Sky News.