The pack of mutant black squirrels that are giving Britain's grey population a taste of their own medicine

But now the black squirrel has arrived - and is rampaging through parks and woodlands.

Scientists say the testosterone-charged black is fitter, faster and more fiercely competitive than both reds or greys.

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Cute but deadly: They may look harmless, but 'mutant' squirrels have left the grey squirrel population in fear

It has already taken over in parts of England and appears to be spreading.

Its rise means the greys now have serious competition for the first time since they were introduced to Britain from America in the 1870s.

The black squirrel is also likely to make life even harder for our native red squirrels.

A study by Cambridge scientists shows that black squirrels now make up half the squirrel population in some parts of the UK.

The upstarts are genetic mutations of greys, but have a darker fur and higher levels of the male sex hormone testosterone - making them more aggressive and more successful.

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Making a run for it: There are fears the black squirrels could wipe out the greys

Geneticist Helen McRobie and Dr Alison Thomas, a professor of life sciences, both from Anglia Ruskin University, Cambridge, conducted the first UK study of black squirrels.

"The population is expanding across the region and is spreading further every year," said Dr McRobie.

"They could overrun most of the Eastern counties within ten years."

The black squirrel is the result of a single mutation in the DNA of greys, giving them higher levels of the pigment melanin in their fur.

Sex selection is also boosting their numbers because female greys appear to prefer them as mates.

Dr Thomas, who completed the year-long study last week, used DNA samples taken from squirrels across East Anglia.

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Fury fight: Red, grey and black squirrels

She said: "There has been a recent population boom. They are due to overtake the grey squirrel population in some parts of the country.

"The first sighting was in 1912, but sightings were very rare until the last few years. They were first spotted around Letchworth Garden City, near Royston, Hertfordshire - currently home to the UK's largest colony.

"But since then they have migrated northwards, penetrating Cambridge city boundaries in the 1990s. Now they make up 50 per cent of the squirrel population in the villages around Cambridge and they are spilling into Bedfordshire and Huntingdonshire."

The rise of the black is the biggest change in squirrel demographics since the indigenous red squirrels almost disappeared 50 years ago from most of England.

Lindsey Maguire, 50, who has co-ordinated the National Squirrel Rescue team for the last 12 years, agreed that blacks were becoming a more common sight.

He added: "The irony of it is the blacks were first spotted in a similar area to where the greys were introduced to Britain.

"Now they may well get their just deserts. I wonder how long it would be before we see a "save the grey" campaign."