It was the sort of unremarkable scene that unfolds every morning in back gardens across Britain. As soon as he opened the curtains, Martin Coats saw a squirrel run the length of his garden wall, past his children's Wendy house and over the fence into the fir trees at the back.

Mr Coats, 36, was reassessing the encounter yesterday after conservation groups confirmed that his street was in the front line of the war against grey squirrels. “It did occur to me that I'd never seen a grey up here before, but I didn't realise the significance,” he said. “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 yesterday that it had received four sightings of a grey squirrel amid the broad crescents and perfectly manicured gardens of the affluent Milton of Leys housing estate near Inverness. These are the first confirmed sightings of a grey in the Highlands, which until now had been Britain's main stronghold of the native red squirrel — Sciurus vulgaris — and is home to at least half of the estimated 120,000 in Scotland.

At a time of mounting concern over the future of the red squirrel population, Ian Collier, woodland officer for Forestry Commission Scotland, said that the invader could have a devastating effect if it carried the squirrel parapox virus — a disease that appears not to affect greys but can be lethal to its red cousins. Mr Collier said: “All it needs is one grey squirrel bringing the virus up - that is the danger and we are very worried about it.”

Ron Macdonald, head of policy and advice at Scottish Natural Heritage, said: “This is one of the areas that has until now been grey-free and has previously been a stronghold for the native red squirrel.”

Although red squirrels do not normally venture into back gardens in Milton of Leys, there is a thriving population in woods bordering the estate, where regular “red squirrel walks” are held by local nature groups. Conservationists fear that the grey may have already reached these woods but the evidence suggests that there is only one of them.

Given the estate's proximity to the main A9 road between central Scotland and Inverness, they suspect that it may have “hitched” a lift on a lorry.

Residents in Milton of Leys were keeping a watchful eye on their back gardens after receiving leaflets asking them to report any sightings.

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.” Finding the invader may not be that easy. Simon Pepper, 43, who saw the squirrel in his back garden three weeks ago, said: “It ran along a 50-metre wall in my garden and then it was gone. It didn't register with me. It was only once I thought about it that I realised it shouldn't be there.”

The native red squirrel has been in retreat across Britain since American greys were introduced in the 19th century, bringing disease and out-competing them for food. Reds have disappeared in southern England except for a few on the Isle of Wight and two small islands in Poole Harbour.

Efforts are under way in Aberdeen to prevent grey squirrels from spreading outside the city and towards the Highlands. It was disclosed this week that an outbreak of squirrel pox had hit an estate in Dumfries and Galloway.

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