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 reportedly investigating the suspected case of H5N6 avian influenza, according to The Sun.
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.