Japan lashed out at anti-whaling activists whose futuristic powerboat had its bow sheared off in a collision in the Antarctic, accusing them of "extremely dangerous" behaviour.
The Ady Gil trimaran came abnormally close to the Japanese whaling ship Shonan Maru No 2 and suddenly slowed down while crossing in front of it, the Fisheries Agency said in a statement.
"These acts of sabotage that threaten our country's whaling ships and crew were extremely dangerous," it said.
"It is totally unforgivable."
There was no major damage to the Japanese ship and no Japanese crew members were injured in the collision, it said.
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All six crew on the Ady Gil, which holds the round-the-world record, were rescued unharmed.
Sea Shepherd animal rights activists have harassed the Japanese fleet over the past six hunting seasons and claim to have saved the lives of hundreds of whales.
An international moratorium on commercial whaling was imposed in 1986 but Japan kills hundreds each year using a loophole that allows "lethal research" on the animals.
Japan makes no secret of the fact that whale meat ends up on dinner tables, and accuses Western nations of not respecting its culture.
