Police are investigating the poisoning of one of Britain's rarest birds, a golden eagle.

The killing of the majestic bird of prey in the Scottish Highlands was doubly illegal - it is strictly protected by law, and it was poisoned by the use of carbofuran, a banned pesticide, probably contained in bait.

The eagle's body was discovered by a hillwalker between Banchory and Braemar in Aberdeenshire, and the presence of carbofuran was later confirmed by tests at the Scottish Agricultural Science Agency in Edinburgh.

Grampian Police said yesterday they were treating the incident "very seriously".

"All crimes against wildlife are disappointing but this incident is even more so due to the species involved," said Grampian Police's wildlife crime officer. Constable David MacKinnon said: "Based on the low population of these specially protected birds, this incident will have a significant effect on the population."

There are only 430 breeding pairs of golden eagles in Scotland, of which about 18 are in the Grampian area.

PC MacKinnon said some gamekeepers were known to use the illegal poison to kill birds such as buzzards or crows, who steal the eggs of grouse and other game birds.

"There will be people who will know what has been going on and we will investigate this rigorously," he said. "The birds will certainly be feeding young and at the time the eagle was found it would have potentially had chicks. "One eagle normally goes to get food while the other sits on the nest and if there is no food coming back then the other bird will take flight."

Police are now appealing to the public, particularly land-based workers, hillwalkers, ramblers and bird watchers to report anything suspicious, including dead birds of prey and any poisoned baits. According to the Royal Society for the Protection of Birds (RSPB), at least 14 golden eagles have been poisoned in Scotland in the past 10 years.

"We think that figure is the tip of the iceberg," said an RSPB spokesman.

Dr Ken Hunter, of the Scottish Agricultural Science Agency, said the number of wildlife poisoning incidents in Scotland this year was "relatively high".

He said the agency had already recorded 20 incidents in 2006, which was more than the whole of 2005, when 19 incidents were logged.

"These are the levels of confirmed incidents, and they don't necessarily reflect all the cases that take place," he added.

Under the Nature Conservation (Scotland) Act 2004, poisoning birds of prey can carry a six-month jail sentence or £5,000 fine.

Dave Dick, a senior investigations officer with RSPB Scotland, said: "These are not victimless crimes and it is not just a few conservationists who get upset when eagles get killed.

"Golden eagles attract tourists and anybody poisoning them is helping wreck the tourist economies of these areas.

"It is a shocking thing to do."