Immigrant animals have been accused of threatening Britain's countryside - but some have been here so long people forget they are the bad guys.

The "invasive mammals" are being blamed for a number of problems from eating crops to threatening native wildlife, a study from the Peoples' Trust for Endangered Species says.

Two of our fastest declining native species, the red squirrel and the water vole, are both under threat from mammals introduced by humans in the last two centuries.

The irony is that some well-known "villains", such as the American mink, the muntjac deer, rabbits and rats, have been here so long that we think of them as indigenous.

The American mink, released from fur farms since the 1920s, eats water voles, which have seen numbers fall by 90% in recent years.

And the grey squirrel, brought to this country from the US in the 19th century, out-competes its smaller red cousin for food and habitat and carries the squirrelpox virus which is fatal to our native species.

The grey squirrel, which also damages trees and forestry plantations, is considered so destructive that it features on the global list of the "worst 100" invasive species around the world.

The People's Trust's wants to find a way of protecting Britain's native animals.

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