One of the Queen's swans was apparently "barbecued" and left on a river bank near Windsor Castle, an animal charity has said.
The bird was butchered and stripped of its flesh before its carcass was dumped close to the water on Baths Island in Berkshire.
Wendy Hermon, 46, of the charity Swan Lifeline which cares for sick and injured birds, said she discovered the "sickening" scene on Sunday afternoon.
"It was just a carcass, it was all burnt. We could see that whoever did this had taken the breast out," she said.
"It was done neatly, presumably to get at the meat. They had skinned it as well and possibly barbecued it there, on a disposable barbecue.
"We have no idea how it was killed, it could have been shot or beaten.
"I felt sick when I saw it. How can someone do that and leave it where they did? I would have been devastated if I had been walking along there with my little boy and he had seen that.
"It just sickens me that there are people out there that do things like that."
The charity worker said she was called to the river bank by a council warden who discovered the swan's remains on Sunday.
The bird is believed to have been killed on Saturday evening.
All wild mute swans in Britain are considered to be the property of the Crown although The Queen only exercises her ownership on certain stretches of the Thames and its surrounding tributaries. It is an offence to kill one.
A Thames Valley Police spokesman said: "Thames Valley Police is investigating a theft following a report at 12.38pm on August 18 by a Royal Borough of Windsor and Maidenhead Council warden who had found a swan. The swan had been killed and burnt."
Killing or injuring a swan used to be classed as treason under a law dating back to the 12th century when the Crown claimed ownership of the birds.
Centuries ago, their meat was considered a delicacy and was served at banquets.
Swans now have statutory protection under the Wildlife and Countryside Act 1981.