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The grey squirrel could become overrun in parts of Britain within 10 years by its faster, fitter and more aggressive black counterpart, researchers in Cambridge said yesterday.

More than 200 years since the grey was first introduced to Britain and began forcing out the native red squirrel, it now finds itself up against a more superior incarnation of the species.

The black squirrel - a genetic mutation of the grey - was first recorded in Britain 90 years ago but now accounts for half of all squirrels in some parts of the country.

They have higher testosterone levels, which is thought to make them more domineering and territorial, while their colouring makes them more attractive to female greys.

Their population growth in recent years is the greatest shift in squirrel demographics since red squirrels were effectively wiped out in 1958.

Helen McRobie, a geneticist, and Alison Thomas, a professor of life sciences, both from Anglia Ruskin University in Cambridge, carried out the first nationwide study of black squirrels.

"The population is expanding across the region and is spreading further every year," said Mrs McRobie. "They could overrun most of the eastern counties within 10 years. They can interbreed with grey squirrels as they are from the same species, unlike red squirrels. You can then have either a black squirrel or a brown mixture."

She said the black squirrel represented a further threat to the red. "The small pockets of red squirrels that still exist already have to be protected because of the grey's dominance," she said. "The black could be even worse. They will eat anything - insects, worms, you name it. The red will eat only matured nuts. They just can't compete."

Miss Thomas said: "The switch from a delicate, silver-toned coat to a dense black pelt occurs as a result of melanin pigments deposited in them during their development. A single genetic mutation can upset this balance, resulting in the over-production of the darker pigment and the birth of a black squirrel."

Miss Thomas said the first sighting of black squirrels occurred in 1912 and had been "very rare" until the last few years. "There has been a population boom and they are due to overtake the grey squirrel population in some parts of the country," she said,

"They now make up 50 per cent of the squirrel population in the villages surrounding Cambridge and they are spilling over into Bedfordshire and Huntingdonshire."

Lindsey Maguire, 50, a co-ordinator with the National Squirrel Rescue team, said black squirrels were an increasingly common sight and it may be that the greys will get their "just desserts.

"I wonder how long it will be before we see a 'save the grey' campaign," she said.

The red: native under threat on two fronts

A native red squirrel

The red squirrel has been native to Britain for 10,000 years but now there are only 120,000 left.

Solitary animals until it comes to breeding, they have been squeezed out by grey squirrels, who eat a more varied diet.

Red squirrels are known to select their food carefully - eating only matured nuts, pine cones, small seeds and fungi - and prefer to live in conifer and Scot's Pine forests.

They suffered badly from the squirrelpox virus and have no immunity to the disease, which is carried by greys, and can die within weeks.

Red squirrels are considered a protected native species and refuges have been set up in Cumbria, the North West and Scotland

The black: an aggressive takeover

The black squirrel

With its more aggressive personality, the black squirrel is becoming increasingly dominant among squirrel numbers in Britain.

First spotted in 1912, there are now an estimated 25,000 centred around the east of England.

A genetic mutation of the grey, it is missing part of the usual DNA sequence that explains its jet black colour.

The black squirrel has a much more varied diet than both the grey and the red.

Its superior physical attributes mean it is starting to overtake the grey.

In Girton, Cambs, black squirrels account for 75 per cent of the squirrel population.

The grey invader whose time may be up

A grey squirrel

The grey squirrel, native to eastern and mid-west America and southern Canada, was first recorded in Britain in the 1820s and formally introduced in 1876 when a pair was released in Cheshire.

A second pair was released at Loch Long in 1892 and, within 25 years, grey squirrels had spread to the eastern side of Loch Lomond and into Stirlingshire.

There are now more than two million grey squirrels across the country and they have virtually wiped out the native red but for the first time it looks like their dominance on British soil may be challenged.