The survey dates back to the 12th Century

The warning comes as Queen's Swan Marker David Barber prepares to undertake the annual, week-long trip up the River Thames to take a census of the swan population.

Mr Barber said: "There have been many reported fatalities, a high proportion of which have been the result of mink attacks.

"Mink are on the increase throughout the Thames area and young cygnets are extremely vulnerable to attack."

Swan Upping, the name given to the annual census, was expected to show a decline in the swan population due to a number of factors.

Closing the circle of boats around the swans

Mr Barber said: "Many reports have been received of vandalism this year.

"The majority of incidents involve the destruction of nests and eggs.

"Fishing hooks and line are a significant factor in the injuries and deaths of many young cygnets.

"Discarded fishing tackle poses a severe threat of drowning, and ingested hooks and line cause immense suffering which often results in death."

Mr Barber says the swan population is in decline

The five-day swan census is due to start at Sunbury-on-Thames and end at Abingdon, Oxfordshire on Friday.

The ceremony dates from the 12th Century when the Crown claimed ownership of all mute swans.

Nowadays, the Crown retains the right to ownership on some stretches of the River Thames and tributaries.

Ownership is shared with the Vinters' and Dyers' Companies which were both granted rights by the Crown in the 15th Century.

The Queen's Swan Marker, accompanied by the Swan Uppers of the livery companies, use six traditional Thames rowing skiffs.

Cygnets are weighed, measured and checked for injury.