At least 65 long-finned pilot whales have died after being stranded on an island off the north coast of Scotland, a rescue charity said on Thursday, in one of the largest mass strandings in Britain in recent times.

The British Divers Marine Life Rescue said it had been alerted to the stranding earlier in the day and sent medics to a beach on Sanday, a Scottish island in the Orkney archipelago.

"On arrival, the medics found there to be about 77 animals high up the beach, having evidently been stranded for several hours already. Sadly, only 12 of them (were) still alive at this point," the charity said in a statement.

Whales can get stranded on shore for a range of reasons, such as when they lose their way or get trapped by tides, but scientists say there is no single definitive reason behind the phenomenon, which has been recorded throughout history.

Pilot whales, in particular, have close social bonds and when one member of a pod gets into difficulties others often follow them, resulting in mass strandings.

Almost a year ago a similar event involving pilot whales occurred on Lewis, another Scottish island located to the west of the mainland, when at least 55 whales died or were euthanised. A mass stranding also occurred in Western Australia earlier this year.