Sea Shepherd threatened last week to escalate its protests and ram Japanese whalers with its ship Farley Mowat, but founder and captain Paul Watson said in a statement received Tuesday that he had been forced to quit.

"We are disappointed to have to leave, but we now have no alternative, as we no longer have the fuel resources to stay," Watson said.

"We have overstretched our fuel and now have just enough to reach the nearest port."

Sea Shepherd had made arrangements to refuel from a tanker near the French Kerguelen Islands but the delivery was cancelled, he said, without elaborating.

Watson threatened to return to the anti-whaling struggle with a new, faster ship next year.

"This year we have kept them on the run and they ran from us like cowards. We intimidated them," he said.

"We knew we could not outrun them; we were limited to chasing them. Next year it is our plan to return with a ship that can match the speed of the Nisshin Maru (the Japanese factory ship).

"If we can keep up with the outlaw whalers, we should be able to prevent them from killing whales every day."

Watson, a founder member of the environmental group Greenpeace, left the organisation in 1977 after disagreements over tactics and has taken a more aggressive approach with Sea Shepherd.

Greenpeace has two ships harassing the Japanese whaling fleet in the Southern Ocean, and has said it has enough fuel to continue its protests for several weeks.

The International Whaling Commission imposed a moratorium on commercial whaling in 1986 but Japan has continued hunting for what it calls scientific research -- a justification rejected by critics.

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