According to Natural England, external, causes for the decline include the introduction of grey squirrels from the USA and Canada from 1876, spreading the squirrelpox virus, which is fatal to the red squirrels only.
The group said habitat loss and weak enforcement of existing protections were adding to the problem.
A spokesperson for the Department for Environment, Food and Rural Affairs, said the government was protecting endangered red squirrels by tackling the threat of non-native grey squirrels.
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