Grey squirrels are killing off their rare red counterparts with a deadly virus in the Lake District.

A population of 30 red squirrels living in Grasmere in the Lake District has fallen to 12 after grey squirrels managed to infiltrate their mountainous terrain.

As reported by The Sunday Times, squirrel rangers patrol the valley and shoot grey invaders.

Yet the Animal and Plant Health agency found a population boom meant it was impossible to control grey squirrel numbers in the isolated area.

“Record numbers of grey squirrels overwhelmed control methods in the valley and then entered the Grasmere area,” the report read.

“Only a few months after grey squirrels entered the area, the first cases of pox virus occurred in red squirrels.”

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Grey squirrels were imported from America by 19th-century landowners to "add variety" to UK wildlife, but they carry a virus which has decimated the native red squirrel population.

While there are now around 2.5 million grey squirrels, there are only a few thousands reds in England and Scotland.

Most red squirrels are contained in isolated areas like Mersea Island in Essex, the Isle of Wight, areas of Northumberland and the Lake District.