The Sea Shepherd campaign says Japan's scientific whaling programme will soon have to close because of financial problems.
Sea Shepherd expects to sail on Tuesday for its seventh campaign against the whaling fleet.
Founder Paul Watson says Sea Shepherd is growing in strength and hurting the whaling programme financially.
Japan's Institute for Cetacean Research says Mr Watson has plucked the figures out of the air and the programme is sustainable, catching 400 whales a year.
The Japanese fleet left last week without a supply ship, but the institute says it will definitely need one for refuelling.