Queen Elizabeth is said to be "profoundly upset" after seven of her swans are reported to have died from bird flu.
The 91-year-old royal is awaiting test results to see if the majestic birds, which are protected under the monarch's royal prerogative, at Windsor were killed by he H5N6 avian influenza, which is believed to have taken the lives of a total of 13 birds.
According to The Sun newspaper, the Department for Environment, Food & Rural Affairs are concerned the infectious avian virus could spread.
A source said: "Bird flu is feared to have struck the Windsor swan flock. They are waiting for the tests to come back but everyone suspects the worst. The results are expected early next week."
Whilst royal author Penny Junor commented: "The Queen will be profoundly upset about this.
"She is an animal lover and if they have bird flu it's horrible."
Royal Swan Marker, David Barber - who advises the country on swan welfare and incidents involving the birds - added: "We are deeply saddened by the loss."
It comes as a double blow for Elizabeth's swans, after six were stabbed and beheaded, after they were found mutilated in Birchmere Park, Thamesmead, south-east London, in November.
The RSPCA setup 24-hour patrols, with the help of locals, around the area to help catch the violent killers.
Nick Wheelhouse, RSPCA inspector, told the BBC at the time: "It's really concerning because the birds seem to have been stabbed and then beheaded, with their bodies left beside the water. While it's a serious animal welfare concern it's also really upsetting for any of the park users who make the grisly find."
The Metropolitan Police helped the RSPCA with the investigation as under the Wildlife and Countryside Act 1981, anyone prosecuted for deliberately trying to injure, kill or take a wild bird can face up to six months in prison and a £5,000 fine.
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