he fururistic protest boat struck by a Japanese harpoon vessel near Antarctica finally sank today as activists accused the Japanese whaling crew of ignoring distress calls.

The hardline Sea Shepherd Conservation Society's powerboat Ady Gil had its bow sliced off on Wednesday by the Japanese security ship Shonan Maru No. 2.

The boat floated in the Southern Ocean for two days as anti-whaling protesters tried to get it to safety at a French research base on the Antarctic coast.

"We lost the Ady Gil about 3 a.m. this morning," Sea Shepherd Captain Paul Watson said.

Watson said Japanese whalers ignored all distress calls after the boat was crippled in the collision with the Shonan Maru, with the six crew picked up by a second Sea Shepherd boat nearby.

"They were responsible, they destroyed the vessel and the effort to try and keep it from polluting the ocean," Watson told Australian state radio.

"I think they should have offered some sort of assistance but they didn't. They refused to acknowledge any distress signal."

Japan's government has previously branded Watson an "eco-terrorist". Two Sea Shepherd activists were confined on a harpoon boat in 2008 after boarding it in freezing waters to carry out protest actions.

Tokyo has lodged a protest with New Zealand's government over the incident, accusing the Ady Gil of suddenly slowing in front of the whaler, causing a collision.

Both Australia and New Zealand have launched inquiries into the clash and have appealed for calm on both sides.

Confrontations between whalers and opponents have become an annual feature of the hunt in Antarctic waters claimed by Australia but not recognised as Australian by Japan.