An Icelandic whaling company is under fire for the horrific accusation that it has killed an endangered blue whale – the first in 40 years.

Sea Shepherd, an international anti-whaling activist group, said the company Hvalur hf, owned by Icelandic multi-millionaire Kristján Loftsson, had killed the animal during the night of July 7, which had since been butchered to be exported to Japan.

“This man must be stopped from ruthlessly violating international conservation law and bringing such disrepute to the nation of Iceland,” Captain Paul Watson, founder of Sea Shepherd, said in a statement. “There can be no legal justification for this crime.”

Commercial whaling has been banned since 1986 after the International Whaling Commission (IWC) issued a global moratorium. However, in recent years Iceland has still allowed its whalers to hunt, letting them catch hundreds of fin and minke whales. Norway, Japan, and others too have flouted the ban.

Blue whales, which are listed as endangered by the International Union for Conservation of Nature (IUCN), have been protected worldwide since 1966. The last recorded catch of a blue whale was off the coast of Spain in 1978.

However, the identity of this whale caught by Hvalur hf remains up for debate. Speaking to CNN, Loftsson said it was either a fin whale or a hybrid species, possibly a rare fin-blue, and not a blue whale.

"We have never caught a blue whale in our waters since they were protected," he said. "We see them in the ocean. When you approach a blue whale, it's so distinct that you leave it alone."

But experts disagree. Adam Pack from the University of Hawaii at Hilo told CNN the hooked dorsal fin, pointed pectoral fins, and size of the animal suggested it was a blue whale. And Dr Philip Clapham from the NOAA Alaska Fisheries Science Center agreed.

"While I can't entirely rule out the possibility that this is a hybrid, I don't see any characteristics that would suggest that,” he told Sea Shepherd. “From the photos, it has all the characteristics of a blue whale; given that – notably the coloration pattern – there is almost no possibility that an experienced observer would have misidentified it as anything else at sea."

Even if this is not a blue whale, it still highlights the fact that Iceland and other countries continue to operate in defiance of international law. Iceland has now successfully done so since 2006, and has been exporting whale meat to Japan since 2008.

"It is unforgivable. We hope that the Icelandic public give their whaling group a hard time about this,” Nicola Beynon from Humane Society International (HIS) told ABC News. “The cruel and inhumane practice of commercial whaling does not belong in the 21st century, it's time to give it up."

Sea Shepherd added to IFLScience that they had asked the Icelandic government to perform a DNA test on the animal, to verify its identity.