Tony Jenkins, from South Norwood Animal Rescue and Liberty (SNARL), based in South Norwood, said the dead cat was discovered on Thursday in the garden next door to its owners in the Southsea area.
Today he told The News he believed the cat was attacked using a blade, and could have been killed by the same person police are looking for connection with several other killings around the country.
Other felines have been found killed in the same way around the London area, within the M25, and as far afield as Manchester and Sheffield.
Mr Jenkins said the charity had received reports of more than 200 cats killed in the same way across the country.
He added: ‘We do not know for certain that it is the same person but the injuries are remarkably similar to what we have seen on other victims around the London area.
‘We have seen so many in the last 15 months. Part of the problem has been that this has been going on for several years but often the police have not investigated.’
He said if the cats were killed by the same person, their job might take them to different places around the country, including Portsmouth.
The cat, a Tortoiseshell, was taken from the garden to the charity’s base in Croydon.
The Metropolitan Police began investigating the killings in 2015, under the name Operation Takahe.
Officers had looked into whether the cats were killed by foxes but Mr Jenkins said it was more likely the cats were attacked by a human.
In February last year the animal rights charity PETA offered a £5,000 reward to anyone providing information to police that leads to the arrest and conviction of the killer, or killers.
Mr Jenkins urged cat owners in the Portsmouth area to stay vigilant, and to report anything suspicious they see to police.
Both Hampshire Constabulary and the RSPCA said they could not find a record of staff attending the incident on Thursday.