The last stronghold of the native red squirrel is under attack in the North, while black “super squirrels” have begun a rearguard assault on their grey rivals in the South.

It was the sort of scene that unfolds every morning in British back gardens. As soon as he opened the curtains Martin Coats saw a squirrel run along his garden wall and over the fence into the fir trees at the back. Though he thought little of it at the time, conservation groups confirmed that his street is now in the front line of the war against grey squirrels.

“It did occur to me at the time that I’d never seen a grey up here before but I didn’t realise the significance,” said Mr Coats, 36. “If it comes back I’ll be better prepared. I’ll get the gun out and, if I see it, I’ll shoot it.” Scottish Natural Heritage said that it had received four sightings of a grey squirrel in the Milton of Leys housing estate near Inverness. It is the first confirmed sighting of a grey in the Highlands, stronghold of the red squirrel and is home to at least half of the estimated 120,000 in Scotland.

Ian Collier, woodland officer for Forestry Commission Scotland, gave warning that the invader could have a devastating effect if it is carrying the squirrel parapox virus — a disease that appears not to affect greys but can be lethal to its red cousins.

Although red squirrels do not normally venture into back gardens in Milton of Leys there is a thriving population in woods bordering the estate. Conservationists fear that the grey may already have made it to these woods but the good news is that all the evidence suggests that there is only one of them. Mr Collier said: “Once we have pinned down the grey’s location we can set live traps to capture it and we would then humanely dispatch it.”Red squirrels are extinct in southern England except for a few on the Isle of Wight and two small islands in Poole Harbour.

But history may be repeating as the greys, an American species introduced to Britain in the 19th century, are struggling against more aggressive black squirrels. These are thought to be fitter and stronger than the greys. They first appeared in Letchworth, Hertfordshire, and have spread to Bedfordshire and Cambridgeshire, and are now heading towards Suffolk. There are estimated to be about 25,000.

In parts of the countryside, especially around Letchworth and Hitchin, they have managed to reach parity with their grey relations and are forecast to take exclusive control.The mutation that gave rise to the black squirrel was identified by researchers from Anglia Ruskin University in Cambridge.

Alison Thomas said: “There’s a lot of observational evidence that the black squirrels are a little bit more aggressive than the greys so when it comes to mate competition it could turn out they are more successful.”

Lindsey Maguire, who coordinates a National Squirrel Rescue team, said: “The irony is that the blacks were first spotted in a similar area to where the greys were introduced to Britain.”

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