The Royal Society for the Prevention of Cruelty to Animals (RSPCA) has launched an investigation after mutilated bodies of at least six swans, including three cygnets, were found in Thamesmead suburb in south London.

Reports suggest two stabbed and beheaded bodies of wild mute swans were found in Birchmere Park while another was discovered at Southmere Lake. Sky News reported that three cygnets and one Canada goose may also have suffered the same gruesome fate after they were reported missing in the area.

Inspector Nick Wheelhouse of RSPCA said the "upsetting" deaths of the birds were discovered in November.

"Park wardens have informed me that they're aware of the deaths of three swans, three cygnets and one Canada Goose so far in recent weeks. It's really concerning because the birds seem to have been stabbed and then beheaded, with their bodies left beside the water," Wheelhouse reportedly said.

"While it's a serious animal welfare concern it's also really upsetting for any of the park users who make the grisly find."

He added that patrolling has been increased in the parks following the shocking discoveries. "Peabody Neighbourhood wardens are now patrolling the area. And locals are so concerned about the birds that they've set up a patrol group to protect them, with various volunteers patrolling the area at night."

Killing wild mute swans is illegal in Britain as they are considered to be the property of the Crown. Anyone found injuring or taking the wild bird can face up to six months in prison and/or a fine of up to £5,000 ($6,714) under the Wildlife and Countryside Act 1981.

Meanwhile, an RSPCA spokeswoman, commenting on speculation that the swan deaths could be related to the mysterious cat killing incidents reported in Croydon, England, said that the possibility could not be ruled out. However, there was no evidence so far pointing to a connection with the cat killer.

More than 360 cats, foxes and rabbits have reportedly been found mutilated in Greater London since September 2015, with many bodies having the heads and limbs removed.

For the past two years, Scotland Yard has reportedly been investigating the murders under Operation Takahe, working closely with the RSPCA and South Norwood Animal Rescue Liberty (SNARL) — an animal rescue organisation. No arrests have been made, Sky News reported.