More than 20 of the Queen's swans from her Windsor flock are feared to have died from bird flu.
The monarch, who technically owns all unmarked mute swans in open water in Britain, is being kept informed.
David Barber, The Queen's Swan Marker, said 20 more of the wild flock, which averages around 200 birds, were seriously ill and also expected to die.
Scientists from the Department for Environment, Food and Rural Affairs are investigating the suspected case of H5N6 avian influenza, The Sun revealed.
Mr Barber said results from the tests on the dead birds from the Windsor flock along the River Thames were expected to confirm bird flu.
"Over 20 swans have died now and there's a lot which are seriously ill which will die - well over 20 more," Mr Barber added.
"We are deeply saddened by the loss of these swans."
He added: "We're waiting to hear from Defra, but yes it is bird flu.
"I'm sure the Queen would be concerned.
"You cannot do anything about it. It will burn itself out. They're wild birds and it is spread by wild birds and we've been very unlucky."
Defra confirmed that seven of the swans that died were being tested for bird flu.
A traditional annual stock-take of swans on the River Thames is carried out each summer.
Known as Swan Upping, the ceremony dates back to the 12th century when the ownership of all unmarked mute swans in open water in Britain was claimed by the Crown in order to ensure a ready supply for feasts.
Today, the Queen exercises this right only on certain stretches of the River Thames and surrounding tributaries.
The ownership is shared with the Worshipful Company of Vintners and the Worshipful Company of Dyers, who were granted rights of ownership by the Crown in the 15th century.
Swan Upping now serves as an annual health check when swans and cygnets are weighed, ringed and checked for signs of disease or injury.
Figures from Defra up to January 30, show there have been six findings of bird flu in wild birds in England, but none in Wales, Scotland or Northern Ireland, despite widespread testing.