Anti-whaling activist Pete Bethune faces a possible jail term when he is sentenced tomorrow in Tokyo for his high seas protest against Japanese whaling.

The Aucklander pleaded guilty in Tokyo District Court last month to charges relating the illegal boarding of a Japanese whaling ship, but not guilty to assault.

A former member of the Sea Shepherd Conservation Society, he has been held in custody since February after he boarded the Japanese whaling fleet's security ship the Shonan Maru II during its annual trip south.

He was captain of the US-based environmentalist group's futuristic powerboat, the Ady Gil, which sank after an earlier collision with the Shonan Maru II.

Sea Shepherd claimed that as a New Zealand citizen, Bethune was entitled to serve a citizen's arrest warrant to the captain of the Shonan Maru II for the sinking of his vessel and the attempted murder of himself and his crew.

The prosecution have demanded a two-year prison sentence, according to the Japan Times.

A Ministry of Foreign Affairs spokesman said that a government official would attend the sentencing.