Seven of the dead birds have been sent for testing to establish whether avian flu is responsible, the BBC reported.
The Department for Environment, Food and Rural Affairs (Defra) confirmed it is investigating a suspected finding of avian influenza in wild mute swans in Berkshire.
David Barber, the Queen's Swan Marker, said "quite a percentage" of the flock had died.
He added: "Bird flu has been going round the country... and unfortunately it's hit Windsor."
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"There are many swans that have died in the flock at Windsor which is extremely disappointing.
"I know there is well over 20 that have gone, and probably 20 still dying at the moment. It's quite a percentage," Barber said.
This outbreak comes days after a bird flu prevention zone was established across England to "help prevent the virus spreading."
Meanwhile, 13 wild birds were found dead in Warwickshire and they were confirmed to have the disease.
The Food Standards Agency said bird flu does not pose a food safety risk for UK consumers.
Avian influenza refers to the disease caused by infection with avian (bird) influenza (flu) Type A viruses. These viruses occur naturally among wild aquatic birds worldwide and can infect domestic poultry and other bird and animal species.
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