Japanese prosecutors have asked for two years in prison for New Zealand anti-whaling activist Peter Bethune.

Bethune is on trial in Tokyo for assault and charges relating to his boarding of a Japanese harpoon ship in Antarctic waters.

Prosecutors say rancid butter stink bombs fired by the activist caused chemical burns to the face of a 24-year-old whaler.

Mr Bethune has pleaded guilty to four charges including trespassing, vandalism and holding a knife, but he has denied the assault charge.

Earlier this week the Sea Shepherd Conservation Society cut its links with Mr Bethune after he carried a bow and arrows during confrontations with Japanese whalers in the Southern Ocean.

The group said his decision to take the weapons on board his ship, the Ady Gil, was unacceptable.

The Ady Gil was sunk after a collision with a Japanese whaling ship.

Mr Bethune's estranged wife, Sharyn, has questioned his expulsion from future protests even though the group maintains it has done so to his assist his court case.