DNA tests on a large whale killed off the coast of Iceland have shown the mammal was a hybrid, researchers have said.

The killing of the whale two weeks ago prompted heavy criticism from campaigners, who believed it to be a protected blue whale.

But Iceland's Marine and Freshwater Research Institute (MFRI) released a statement on Thursday saying genetic results proved the creature was a hybrid.

"The results of the genetic results confirm the preliminary assessment that the whale in question that was caught on July 7th was a hybrid of a fin whale father and a blue whale mother," the statement said.

Whaling company Hvalur hf, which reportedly killed the whale, is allowed to hunt fin whales legally in Iceland.

Photos of the whale provided by Icelandic-based anti-whaling group Hard to Port showed the large animal being towed into a port and crew members standing alongside it.

Anti-whaling activist group Sea Shepherd claimed that photographs and video taken by its team on the ground showed it was a blue whale, but this was refuted by Hvalur hf.

Researchers said these hybrids are rare and trading their meat is illegal.

A blue whale - the largest animal to have ever lived - has not been deliberately caught since 1978, according to the International Union for the Conservation of Nature (IUCN).

Iceland resumed commercial whaling in 2006. Although an international moratorium has placed catch limits at zero for blue whales, Iceland, Norway and the Russian Federation all objected to that provision.

During this year's season, Sea Shepherd said Hvalur hf had killed 22 whales.