THE Australian anti-whaling group Sea Shepherd has had flagship the Steve Irwin seized in the North Atlantic.

The marine activists say they urgently need to raise a $1.4 million bond to free the vessel, tied up in a legal stoush over a tuna catch.

Maltese fishing firm Fish and Fish Ltd is seeking damages through British courts over the group's release of 800 prized bluefin tuna from its nets last year.

If the Sea Shepherd Conservation Society does not quickly raise the bond, it fears the vessel could be held indefinitely, or sold.

Not long back from anti-whaling activities in Antarctica, the Steve Irwin was readying to defend pilot whales off Denmark's Faroe Islands when it was detained in the Shetland Islands, part of the UK.

Steve Irwin skipper Paul Watson appealed for public help. "We've already helped save thousands of whales and untold numbers of other marine creatures, but unless we get our ship back soon, whales will die tragic deaths because of our inability to move this ship and protect them," he said.

"If the funds to meet the bond are not raised in time, it will not only take a huge financial toll on the organisation but could also ... threaten our ability to defend whales in the Southern Ocean Whale Sanctuary from the Japanese whaling fleet in December," a spokesman said.

Sea Shepherd plans to contest the litigation, alleging the tuna were illegally caught after the close of the season, without an inspector on board, and without proof of the legality of the catch.

Originally published as Irwin vessel seized to the tuna of $1.4m