The high-tech stealth boat Ady Gil was cut in half and sunk by a Japanese security vessel in Antarctic waters today, dramatically upping the stakes in the annual struggle between whalers and protesters.

Sea Shepherd group leader Paul Watson told Fairfax Media the $1.5 million Ady Gil was sinking, but its six-man crew had been rescued and was uninjured.

Captain Watson said the Ady Gil was idling in waters near Commonwealth Bay,south of Hobart, when it was suddenly approached and rammed by the Japanese ship Shonan Maru, which has been detailed to provide security to the fleet.

Earlier today, the fleet was contacted for the first time by the Ady Gil and Sea Shepherd's "secret" third vessel, the Bob Barker.