Images documenting the slaughter have been captured and have left international whale experts deeply concerned.
Just before midnight on Saturday, the Hvalur hf. fin whaling company landed its 22nd catch of the season at the whaling station at Hvalfjörður, Iceland.
The whale hauled up the slipway caught the attention of Arne Feuerhahn, a German conservationist documenting the hunt.
The whaling company had resumed a controversial hunt for fin whales on 22 June after a two-year break.
The season could see as many as 238 endangered fin whales killed under a self-allocated quota, which defies the international ban on commercial whaling.
However, blue whale experts have studied the images of the creature landed last Sunday and found it was either a male blue whale or a blue/fin hybrid.
Blue whales are the largest creatures on the planet.
Reaching as much as 33m in length, they are listed as endangered.
Their global population is estimated at only 10,000 to 25,000 individuals.
They have been protected from hunting since 1966.
It has been estimated up to 90 per cent of the global population of blues was wiped out by commercial whaling operations.
Blue\fin whale hybrids are relatively rare.
Icelandic authorities confirm five hybrids have been identified by researchers since 1986 around Iceland. Four of these were killed by Icelandic whalers.
Vanessa Williams-Grey, policy manager at charity Whale and Dolphin Conservation (WDC), said: “This incident confirms that whaling is still a serious threat to endangered whales.
“This industry was out of control in the 20th century and it remains so today.”
Arne Feuerhahn, CEO of campaign group Hard to Port, said: “This is an unacceptable tragedy that leaves people around the world speechless. It is very unfortunate that the reckless and irresponsible actions of a single individual stain the reputation of this progressive and beautiful country.”
Blue whale experts globally have urged WDC to contact Icelandic authorities, calling for urgent genetic testing to establish beyond doubt the species involved.