The Queen's swan marker David Barber said this week that the bird flu virus that has hit Windsor could end up claiming a quarter of the swan population.

Officials began to suspect last week that swans might be dying from the lethal avian influenza virus - and at 5pm on Monday this was confirmed after tests were carried out by the Department for Environment, Food and Rural Affairs (Defra).

Mr Barber, whose job sees him travel the Thames annually checking on the health and numbers of swans, admitted that there was little that could be done to combat the current epidemic.

He said: "In all my years as the Queen's swan marker I have never known this on the Thames."

He said that about 30 swans had now died and he feared that in the end a quarter of the swan population might be lost.

He said: "It is an airborne disease. Seagulls and ducks carry it and when they land they pass it onto the poor swans. It usually burns itself out in two to three weeks and there is little we can do."

Wendy Hermon, the treatment co-ordinator with the Swan Support rescue charity, has been at the centre of the crisis.

She said: "Last Monday we had a call to a dead swan. We get calls all the time this time of year so it was not unusual. But then we got calls Tuesday and Wednesday and I saw a swan on the slipway at River Street who looked really poorly - I thought then 'there is something not right here'."

Once the cause of the problem was suspected Wendy and some of her fellow swan rescuers found themselves unable to return to the charity's home in Horton Road, Datchet where their rescue swans are kept, because of the risk of spreading the contagion.

Fortunately the centre has about 15 volunteers, so those who had not attended any of the infected swans have been filling the breach.

Mrs Hermon has continued collecting the growing number of dead swans, now wearing protective clothing approved by Defra.

David Barber praised her work this week.

Mrs Hermon has petitioned the Government calling for a ban on the sale of airguns after a number of swans were shot or injured in recent years. If her petition gets 100,000 signatures, it could be considered for debate in Parliament.