It's feared more than 20 of the Queen's swans, from her Windsor flock, have died from bird flu.
Twenty more are seriously ill and also expected to die.
The monarch, who technically owns all unmarked mute swans in open water in Britain, is being kept informed.
More than 20 swans have died now and there's a lot which are seriously ill which will die - well over 20 more. We are deeply saddened by the loss of these swans."
Defra confirmed that seven of the swans that died were being tested for birdflu.
An annual stock-take, known as Swan Upping, takes place each summer on the River Thames.
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 supply for feasts.
Today, the Queen exercises this right only on certain stretches of the river and surrounding tributaries.
As of the 30th January, there have been six findings of bird flu in wild birds in England.
But none in Wales, Scotland or Northern Ireland.