Militant activists seeking to cripple Japan’s Southern Ocean whaling campaign said they had tracked down and intercepted a factory ship key to the Antarctic harpoon mission.

A vessel from the Sea Shepherd Conservation Society found the Nisshin Maru in the ice of the Ross Sea after a 26-day chase covering 4,000 nautical miles, striking a blow to the Japanese fleet.

“We found it and it had just started whaling again, so we know they killed one whale,” activist Captain Paul Watson said.

“But they’re now running from us and running full speed so they’re not whaling today, that’s for sure.”

Sea Shepherd used weather balloons fitted with radar to zero in on the “serial killing death ship”, which slipped away from the activists when they first found the Japanese fleet on December 31, he added.

While searching for the Nisshin Maru, Mr Watson said his boats – the Steve Irwin, Bob Barker and superfast interceptor craft Gojira – were followed by two of the three Japanese harpoon ships, which prevented them from spearing whales for more than three weeks.

Sea Shepherd also found and intercepted the fleet’s refuelling and supply ship, the Korean-owned Sun Laurel, said Mr Watson, in another blow to the Japanese mission.

“This has been our most successful year so far. We found the whaling fleet before it even begun to kill whales and I don’t think they’ve taken very many whales at all,” Mr Watson remarked.

“We’ve had two of their three harpoon vessels tied up since December 31st, they’re low on fuel and we intercepted and cut off their supply vessel, so I think they’re going to be looking at a big loss.”

The Steve Irwin was now positioned behind the Nisshin Maru’s slipway, blocking them from loading whales, he said.

“That can shut down their entire operation,” he said. “We’ll continue to pursue the Nisshin Maru, and as long as we have them in our sights and they’re running away from us they’re not going to kill any whales.” Japan kills hundreds of whales a year under a loophole in the 1986 moratorium on commercial whaling that allows “lethal research”, but makes no secret of the fact that the meat ends up on dinner tables.

Anti-whaling nations, led by Australia and New Zealand, and environmental groups have long criticised the hunts, describing them as cruel and unnecessary.

Canberra is currently fighting Japan’s whaling programme in the International Court of Justice, seeking for it to be declared illegal.