By Steven McKenzie
Highlands and Islands reporter, BBC Scotland news website
Grey squirrels are a non-native species originating from North America
The first grey squirrel to be officially reported in the Highlands may have arrived on the back of a lorry carrying hay or straw bales.
The region is deemed to be a stronghold for native reds, free of the risk of competition for food and a disease carried by the American species.
Conservation project See Red Highland has raised concerns at the appearance of a grey near Inverness.
The rogue squirrel may be trapped and humanely put down due to the risks.
Ian Collier of See Red Highland - an organisation backed by several public bodies including Forestry Commission Scotland and Scottish Natural Heritage - told the BBC Scotland news website one of the theories on how it arrived was on a transporter.
It is illegal to release the animals into the wild.
Squirrel pox virus
The Highlands grey was spotted in Milton of Leys on the outskirts of Inverness.
The A9 road - the main route into the north from the central belt - runs close to the area suggesting a possible way it reached the area.
Households in Milton of Leys have been asked to look out for the mammal.
Mr Collier said: "This is the first grey to be confirmed in the Highlands.
"The main problem having a grey in the Inverness area is that it may be carrying squirrel pox virus, which is fatal to red squirrels, and that it competes for food."
Ron Macdonald, head of policy and advice at Scottish Natural Heritage, added: "This is one of the areas that have until now been grey-free and has previously been a stronghold for native red squirrel.
"That no longer appears to be the case with this first confirmed sighting of a non-native grey squirrel."
Squirrel survey website
This week tests confirmed cases of the virus at three sites on an estate in the south of Scotland.
The disease, which can be carried by greys without harming them, was found on the Queensberry Estate near Dumfries.
Routine monitoring of greys confirmed the infection.
The non-native species, which was introduced to Britain by the Victorians, has not previously been confirmed in the Highlands.
Strathdon in the heart of Aberdeenshire and Blair Atholl have been the nearest the animals have come to the region's borders.
Highland Red Squirrel Group was established to help monitor and conserve reds.
Mr Macdonald added that SNH was asking people to get in touch if they spotted a grey squirrel or to log any sightings on its squirrel survey website.