Seventy-seven long-finned pilot whales washed ashore on Thursday on Sanday island, a part of the Orkney archipelago off Scotland.
Sixty-five of them were already dead, the British Divers Marine Life Rescue (BDMLR) service said, and the 12 still alive were in too poor a condition to recover.
"Sadly the remaining 12 pilot whales have been euthanized due to their condition deteriorating from the many hours they have spent stranded on the beach," the BDMLR said in an update.
It said lying on the beach for a long time likely led to "crush injury from their own weight and the high likelihood that they have inhaled water with the incoming tide."
The charity said the whales had sunk deeper into the sand as the tide washed over them and were unable to refloat themselves.
No 'obvious indications' for why they were stranded
The Orkney Islands are an archipelago situated just off the northeastern tip of mainland Scotland. Sanday is to the northeast of the group of islands, nearer the open waters.
"There are no obvious indications as to why they all stranded," the BDMLR said, adding that it would try to recover and conduct postmortem examinations on as many as possible.
Such mass strandings are not uncommon among pilot whales, who tend to travel in tightly-knit groups, but scientists are still trying to ascertain why the phenomenon occurs.
"This is definitely one of the larger mass strandings, but not necessarily the largest," the BDMLR told the AFP news agency.
In 2023, about 55 pilot whales were stranded on the island of Lewis, off Scotland's northwest coast.
Larger mass strandings involving the whales are also quite common around Australia and New Zealand.