A deadly new disease introduced into Britain by exotic snakes has attacked and killed some of the country’s native adders and grass snakes, threatening to decimate their numbers.

The National Trust has warned that pet owners who abandon unwanted exotic snakes such as boa constrictors in the countryside have created a fatal new danger for already threatened native species.

Experts have found evidence that a number of native grass snakes have succumbed to Snake Fungal Disease, which causes lesions on the snake's body, leading to infection and death.

In the latest case a 4ft (1.2m) Dumeril’s boa constrictor from Madagascar was found abandoned at Kinver Edge, on the Staffordshire/Worcestershire border.

That followed the discovery of four American Western Hognose snakes at the same beauty spot.

The trust has been looking after Kinver Edge for more than 100 years and its experts have reported that the number of adders and grass snakes at the site is falling.

They say the habit of abandoning exotic snakes can only serve to further threaten the native population of grass snakes and adders, already under threat from urbanisation and loss of habitat.

The pathogen has been found in 23 species of wild snake in the US and at least three in Europe.