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An Antarctic battle of nerves pitting Japanese whalers against radical environmentalists threatens to intensify as two protesters — one British, one Australian — entered their second night locked in the brig of a harpoon boat.

Members of the Sea Shepherd Conservation Society, the group that vowed to “do everything in its power” to hinder the Japanese annual whale hunt, said that they would consider sending a boarding party to rescue their colleagues from captivity.

Each side accused the other of “piracy and terrorism” after the two protesters boarded the Japanese ship, hurled glass bottles of foul-smelling acid on to the deck and were quickly detained by the whaling crew.

The Japanese Government grudgingly admitted yesterday that the protesters may have scored a temporary victory in their attempt to stop the hunt: the activities of the Yushin Maru No 2 harpoon boat will be suspended as long as the two protesters are on board and, for the moment at least, there appears to be little way of returning them to their own vessel.

In what Tokyo regards as a deliberate attempt to prolong the showdown, the ship belonging to the Sea Shepherd group has sailed off the Japanese vessel's radar, making an immediate handover of the protesters impossible. Both sides claim that the other has its radio switched off and is refusing to answer calls.

The Japanese Foreign Ministry yesterday said that it was ready to return the two protesters, Giles Lane, a Briton, and Benjamin Potts, because it accepts that they came on board the Yushin Maru No 2 to deliver an anti-whaling petition. However, a spokesman said that it had received no contact from the crew of the activists' flagship, the Steve Irwin, about how to organise the handover and may not for some days.

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