Australia has given Japan an ultimatum to end its controversial whaling activities.

If Japanese whalers take to the seas when the new season begins in November, Canberra will take the matter to the International Court of Justice, said Prime Minister Kevin Rudd. In doing so he is fulfilling an election promise.

Australia has threatened to take legal steps before, but has never acted on its threats, to the disappointment of environment organisations.

An activist from the Sea Shepherd environment organisation, Peter Bethune, who climbed on board a Japanese whaler on Monday, is being held prisoner. He climbed on board the ship to make a citizen’s arrest after two of the organisation’s boats were rammed by Japanese whalers recently in the southern Antarctic. His boat the Ady Gil sank.

Mr Bethune will be handed over to the Japanese authorities in Tokyo.