Anti-whaling activist Pete Bethune now says he deliberately sank the vessel Ady Gil after it was hit by a Japanese whaling ship earlier this year.
The admission comes during a bitter feud between the activist and his Sea Shepherd Conservation Society Boss Paul Watson.
Bethune has accused Watson of ordering him to sink the boat as a publicity stunt, and quit Sea Shepherd in disgust at the organisation's antics.
The protestor said the Ady Gil was capable of being towed back to port after it was rammed, but instead he was instructed to go inside the engine room and open the seacocks, to allow in the water to flood the boat.
"When Chuck, Luke and myself went to do the scuttling, we were discreetly filmed by one of my crew," Bethune wrote.
"We were filmed going to the Ady Gil ... working to open compartments in the sponsons and rear hatches, and then returning to the Bob Barker."
That night the crew discussed what Bethune had done, how long the sinking would take, and the legal ramifications of what had occurred.
He says other members of the ship's crew were there, and they're prepared to back his allegations.
In a letter to Watson, made public by Sea Shepherd during the row, Bethune wrote that sinking the vessel "broke his heart."
"I felt horrid after the scuttling and I have felt terrible about it ever since. It broke me heart to sink a vessel that had been such a big part of my life," Bethune says.
The Ady Gil, formerly known as Eathrace, was a 24m, high tech trimaran powered by biodiesel, and was originally created as part of a successful project to break the world record for circumnavigating the globe in a powerboat.
Bethune says he has now quit the Sea Shepherd Conservation Society, and is accusing the organisation of deliberately misleading the public.
However, Sea Shepherd says Bethune didn't quit - he was fired for laying the blame on Watson when questioned by the Japanese authorities during his five-month incarceration.
Watson says Bethune's testimony, pointing the finger at him, means there is now a warrant for his arrest in Japan and his name is on the Interpol Blue List, making travel between countries difficult.
In a letter to Bethune, Watson says the protestor's courage faltered under interrogation.
"And because of that we cannot work with you. We cannot promote your book or allow you to be involved in Sea Shepherd activities or to go on Sea Shepherd campaigns. We simply can't trust you not to provide statements that will cause us problems."
Watson said the rule was supposed to be "Nobody talks, everybody walks" except if a prisoner was tortured.
"And before you say that I don't know what it is like to be held in custody, I will tell you that I was held for 120 days on a warrant from Norway. During that time I said absolutely nothing and I was released without charges," Watson said.
He told Radio New Zealand this morning that Bethune had been fired as the organisation wanted to distance itself from him.
Bethune, 45, was deported back to New Zealand after receiving a two-year suspended sentence in July when he was convicted on five charges relating to clashes with Japanese whalers in Antarctic waters, in which he hurled rancid butter and scaled a Japanese vessel.
He spent spent five months in a Japanese prison awaiting the trial.
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