A court in the US has labelled conservationist group Sea Shepherd "pirates".
Judge Alex Kozinski said the group's "aggressive and high-profile attacks" on Japan's whaling fleet endangered lives, ordering them to stop.
US-based Sea Shepherd has for many years chased the Japanese whalers, attempting to disrupt the annual hunt.
The two sides have frequently clashed at sea, blaming each other for collisions and damage.
Three Sea Shepherd ships have been involved a stand-off and clashes with the whaling fleet in the icy waters of the Southern Ocean for several weeks.
They have been trying to prevent the Japanese ships from refuelling from a tanker ship, the Sun Laurel. Both parties released video footage this week which they said showed the other deliberately ramming their ships.
Sea Shepherd has also accused the whalers of using water cannon and stun grenades against them, and says Japan has deployed a military icebreaker, the Shirase, to intimidate them - something Japan rejects.