A Court of Appeal in the United States says Sea Shepherd are pirates and that Japanese whalers have a right to carry out their activities in the Southern Ocean.
Sea Shepherd boats and Japanese whaling ship the Nisshin Maru have clashed at sea again as the organisation tries to disrupt the annual cull.
Chief judge Alex Kozinski said in his decision: "You don't need a peg leg or an eye patch to be a pirate''.
The judge said Sea Shepherd founder Paul Watson is a pirate, no matter how high-minded he believes his cause to be.
The judge overturned a previous decision that effectively allowed Sea Shepherd to continue its campaign in the Southern Ocean.
He said Japan is allowed under international law to carry out whaling for research purposes and he accepts that is what the Nisshin Maru is doing.
But Paul Watson says the American court has no jurisdiction over his Australian ships, which are operating in Australian Antarctic waters.
Mr Watson said the comments would not disrupt Sea Shepherd's campaign to block the Japanese whalers.