Japan's coast guard have arrested an anti-whaling activist from New Zealand for illegally boarding a Japanese whaling ship last month, an official said.

Peter Bethune, a member of the US-based Sea Shepherd activist group, is accused of jumping aboard the whaling vessel from a Jet Ski on February 15 in Antarctic seas, where Japan was conducting its annual whale hunt.

Coast guard spokesman Tomonobu Higashi confirmed Mr Bethune's arrest.

Since the boarding, the 44-year-old New Zealander has been in custody on the Japanese whaling ship. The vessel - the Shonan Maru 2 - returned to Tokyo port Friday from the southern seas.

Under Japanese law, boarding a Japanese vessel without legitimate reasons can bring a prison term of up to three years or a fine up to 100,000 yen (£730). It is the first time Japan has arrested a Sea Shepherd member.

The brazen boarding was the latest incident in Sea Shepherd's campaign to hamper Japanese whaling activities.

The activists trail whaling boats and try to disrupt the hunt by dangling ropes in the water to snarl the ships' propellers and hurling packets of stinking rancid butter on the whaling ships' decks. The whalers have responded by firing water cannons and sonar devices meant to disorient the activists.

Japan described the Sea Shepherd protesters as terrorists, and Mr Bethune's arrest received extensive coverage on Japanese TV. In recent months, Sea Shepherd vessels and Japanese whalers collided twice. One protest vessel - the Ady Gil that Mr Bethune captained - sank in a collision with the Shonan Maru 2 in early February.

Japan's whale hunt is allowed by the International Whaling Commission as a scientific programme, but opponents call it a cover for commercial whaling, which has been banned since 1986.

Coastguard spokesman Masahiro Ichijo said officials have two days to interrogate Mr Bethune before handing him over to prosecutors to decide whether to press formal charges against him to stand trial in Japan. Mr Ichijo said authorities are also considering additional allegations, including assault and destruction of property.