Photographic evidence provided by an Icelandic anti-whaling group appears to show the carcass of a blue whale, the largest animal that has ever lived, being butchered for export in the fjord of Hvalfjordur.
The whaling company owner, Kristjan Loftsson, denied the allegations, insisting the animal was a fin whale or a hybrid of the two, which are not protected under Icelandic law.
But a number of experts concluded the images showed a whale with all the characteristics of a blue.
"It’s definitely a blue or at the very least a blue-fin hybrid, Ellen Coombs, a biologist at University College London, told The Independent, noting that only a handful of hybrids had been spotted in Icelandic waters in the last 30 years.
Ms Coombs, who is researching the impact of climate on whale diversity, said the images showed a much lighter dorsal fin than would be found on a fin whale, with blue-grey mottled skin characteristic of a blue. A white underbelly and chin commonly found on fin whales appeared to be absent as well.
"Now is the best time to see blues around Iceland, so I imagine that despite still being rare, their numbers around Iceland are at their highest during these months," she added.
Phillip Clapham, of the US National Oceanic and Atmospheric Administration (NOAA) Alaska Fisheries Science Centre, said in a statement: "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."
Paul Watson, founder of marine conservation group Sea Shepherd, said: “I know a blue whale when I see one and this whale slaughtered by Kristjan Loftsson is a blue whale.
"This man must be stopped from ruthlessly violating international conservation law and bringing such disrepute to the nation of Iceland. There can be no legal justification for this."
If confirmed as a blue whale, it would be the first known time one had been deliberately captured and killed in 40 years, according to the International Union for the Conservation of Nature.
“We have never caught a blue whale in our waters since they were protected," Mr Loftsson, managing director of Hvalur hf, told CNN. "We see them in the ocean. When you approach a blue whale, it's so distinct that you leave it alone."
He added that if it was a blue whale, the catch was entirely accidental.
The Icelandic government said it was taking the matter “seriously” and that the relevant authorities, which will use DNA testing to confirm the whale’s true identity, were already investigating.
Images of the dead whale provided by anti-whaling group Hard To Port triggered fury among experts and activists.
“So @MFAIceland @IcelandinUK your countrymen have apparently killed an @IUCNRedList Blue Whale thus transgressing environmental laws and morality,” naturalist and broadcaster Chris Packham said. “What do you plan to do next?”
So @MFAIceland @IcelandinUK your countrymen have apparently killed an @IUCNRedList Blue Whale thus transgressing environmental laws and morality . What do you plan to do next ? And what stance will @GOVUK take . Please RT if you would like an outright ban on all whaling now . pic.twitter.com/hHBG8K3x7w— Chris Packham (@ChrisGPackham) July 12, 2018
Bella Lack, a prominent teenager animal welfare activist, said: “Once again humans lack the respect that this magnificent beast deserves.”
“First you (Iceland) allow the hunting of endangered Fin Whales. Now the company hunting the #Whales have killed a critically endangered Blue Whale. Their license should be revoked immediately,” said Daniel Schneider, a biologist and traveller.