The anti-whaling group Sea Shepherd says it has intercepted the Japanese fleet in the Southern Ocean.
The annual whale hunt is expected to begin within days.
Sea Shepherd says its high-speed trimaran the Brigitte Bardot has made contact with the Japanese fleet's harpoon ship, which is en route to the Antarctic whaling grounds.
The whaling fleet left Japan for the Southern Ocean in late December, planning to catch up to 935 Antarctic minke whales and up to 50 fin whales.
Former Australian Greens leader Bob Brown, who is now leading the Sea Shepherd campaign, says it appears the whaling ship has been intercepted before firing a harpoon.
"So we're one day short of the end of January, the prime killing month for these whaling fleets, and they haven't yet been able to kill a whale and Sea Shepherd is very, very happy," he said.
The militant conservation movement has successfully stymied the Japanese hunt in recent years.
This year's campaign is Sea Shepherd's biggest, with the group employing four ships, a helicopter, three drones and more than 100 crew.
Sea Shepherd founder Paul Watson is wanted by Interpol after skipping bail last July in Germany, where he was arrested on Costa Rican charges relating to a high-seas confrontation over shark finning in 2002.
Mr Watson is on board Sea Shepherd's main ship, Steve Irwin, but has stepped down as skipper.
He has vowed to abide by a US court ruling in December banning the group from physically confronting any vessel in the Japanese fleet.
Sea Shepherd says it has saved the lives of 4,000 whales over the past eight whaling seasons, increasing its campaigns of harassment against the Japanese harpoon fleet.
Tokyo claims it catches whales for scientific research - a loophole in the international ban on whaling.