The bodies of the birds were discovered on Friday on a popular dog walking route in Priest End which runs alongside the River Thame.

One body was floating in the water and the other was on the riverbank. An adult swan and another cygnet are healthy and remain on the river.

It is not known how the swans died but RSPCA inspector Martin Rivett has been told that youths had been seen swinging bottles on ropes at the swans from a bridge on Wednesday last week.

He said: "It is hard to tell how the cygnets had died, especially as the one on the bank had since been attacked by another animal.

"However, if they were the targets of such an appalling and deliberate attack, we need to hear from anyone who knows anything about this."

Thame mayor Jeanette Matelot Green said: "I was absolutely horrified to learn of this cruel and deliberate attack. One cannot imagine how anybody could behave in this despicable manner.

"We had a wonderful Festival in Thame on Saturday, followed by a Duck Race at Priest End on Sunday and this awful discovery has tainted what was a marvellous weekend.

"I can only hope the culprits are found soon and punished accordingly."

Anyone found guilty of injuring or killing a protected wild bird could face a maximum six-month prison sentence and/or a £5,000 fine under the Wildlife and Countryside Act, 1981. Anyone with information should contact the RSPCA in confidence on 0300 1234 999.