Japan's whalers have won an injunction in a United States court against Sea Shepherd, restraining the anti-whaling group from attacking their ships in the Southern Ocean.
The decision by the US Ninth Circuit Court of Appeals was described as "somewhat astonishing" by Sea Shepherd leader Paul Watson on Tuesday.
It follows an appeal to the court by Japan's Institute for Cetacean Research against the decision of a US District Court judge refusing an injunction earlier this year.
Chief Judge, Tashima Kozinski said in the decision that Sea Shepherd, Mr Watson, and anyone acting for them, were "enjoined from physically attacking any vessel engaged by Plaintiffs the Institute of Cetacean Research, Kyodo Senpaku Kaisha, Lt., Tomoyuki Ogawa or Toshiyuki Miura in the Southern Ocean."
Kyodo Senpaku Kaisha operates the whaling fleet for the ICR, While Mr Ogawa and Mr Miura are believed to be senior officers.
The order also enjoins any person or vessel from navigating in a manner that is likely to endanger the safe navigation of the whaling fleet.
"In no event shall defendants approach plaintiffs any closer than 500 yards when defendants are navigating on the open sea," it says.
The injunction is to remain in place until the court decides on the merits of the Japanese case opposing Sea Shepherd in Washington State, where the group is headquartered.
Mr Watson said the situation was complex, covering as it did Dutch and Australian registered ships, and an international crew.
"It's somewhat astonishing that a US court can assume jurisdiction over non-US ships in international waters," he said.
He said that despite leaving port and moving around the waters of Japan, it appeared that the Japanese fleet was yet to depart for the Antarctic this year.
"I think just maybe the Japanese fleet was waiting for this order before sailing," he said. "They will find when they arrive that we will stand firm in our defence of the whales."