The British Divers Marine Life Rescue charity said police alerted them to the incident on the Isle of Lewis at about 7am.

“Reports so far suggest there are around 55 animals, however the latest update from the scene indicates that only 10 are alive,” it added in a statement.

Pilot whales are part of the cetacean family which includes dolphins and can grow up to 6m in length and weigh a tonne.

It was not immediately clear why they have become stranded.

Scientists, who have studied the phenomenon, believe some pods go off track after feeding too close to shore.

The BDMLR said they were “notorious for their strong social bonds, so often when one whale gets into difficulty and strands, the rest follow, leading to more of them stranding”, a statement read.

Police Scotland confirmed that its officers were part of the rescue operation at Tolsta, Stornoway.

The maritime and coast guard agency said it had also responded to the incident.

The BDMLR, founded in 1988, trains volunteer marine mammal medics and has 20 whale rescue pontoons at strategic locations across the UK to help stranded whales and dolphins.

Between 2011 and 2015 it responded to a series of mass strandings of pilot whales across Scotland.

The biggest involved a pod of more than 70 animals.