The bodies of about 100 cats have been found stuffed behind radiators and in two freezers in a flat in Leeds.
Dozens of dead cats were found in the flat by workers
RSPCA inspectors were called to the property at Kingsdale Court, Boggart Hill Road, Seacroft, by a company cleaning the block of flats.
Floors throughout the flat were covered in about 2ft of human and cat excrement, the RSPCA said.
Insp Lynsey Harris said: "I was horrified by what I discovered in that property... it's unbelievable."
The animal charity had been called out by the firm A-Z Environmental Services which was cleaning the block.
Inside Insp Harris found 100 dead cats, including animals which had been stuffed behind radiators and crammed into a chest freezer and an upright freezer.
Alongside the bodies in the upright freezer was edible human food.
She said: "The smell was absolutely sickening. It is unbelievable, but it appears that someone may have been living among these corpses. What on earth was going on here?"
Power at the flat had been switched off for some time.
Vets at the charity have said they are unable to carry out a meaningful post-mortem examination on any of the bodies because of their state and inspectors do not know if the animals were killed there or elsewhere.