The RSPCA yesterday defended its decision to prosecute a policeman who put an injured cat out of its misery by smashing it over the head with a spade.

Pc Jonathan Bell thought he was doing the right thing when he killed the animal in April 2004 after it had been run over and its body crushed.

He had been called to another late-night incident when members of the public asked him to see the badly injured cat.

The 36-year-old officer was advised by colleagues the RSPCA was unavailable at that time of day so he borrowed a shovel and dealt the stricken animal a fatal blow. However, a passer-by reported the incident to the association, which began a prosecution of Pc Bell that took two years and cost tens of thousands of pounds.

A district judge acquitted the married father-of-two, saying the officer had done what he thought was right at the time.

And the RSPCA's application for leave to appeal was recently refused by the High Court.

However, the officer went "to hell and back" during the court case and needed time off work because of stress, said Staffordshire Police Federation chairman Mark Judson.

"He thought he was doing a police duty," he said.

"He was certainly not overtly cruel to an animal and he feels that he has been harassed by the RSPCA who would not let it go."

The case has also raised concerns about the animal welfare charity's dual role of bringing private prosecutions and raising its profile to attract donations.

Chris Newman, of the Federation of Companion Animal Societies, said: "I think the issue is that the RSPCA has become too much of a political campaigning body and that prosecutions have been an easy way to raise profile and money."

Pc Bell, speaking after the High Court hearing, said he had "no regrets" about how he dealt with the cat.

He said: "I am happy I killed it with the first blow. I made up my mind that I would hit it a number of times to make sure it was dead. I made the decision on what I had in front of me. I looked at the cat and I could see it was completely crushed at the back. I looked at it for between three and five minutes. I decided to put it out of its misery when I realised I was not going to get help from anywhere else."

Colin Vogel, a vet called to give evidence in the trial as an independent expert witness, said the cat had been squashed to within an inch thick at its lower half.

"He did the kindest thing, which was to put it out of its misery, whereas if he'd just walked away leaving it injured he could have just as easily faced a charge of animal cruelty." Pointing to its 97 per cent success rate in court cases, the RSPCA defended its decision to prosecute Pc Bell. It also denied it had a conflict of interest between its prosecuting and fund-raising roles.

A spokeswoman said: "Clearly we do believe that we should highlight where serious animal welfare crimes are committed because it will hopefully deter others from doing the same."

She said the court case had had a positive outcome because Staffordshire Police had reviewed its procedures where its staff are dealing with injured animals.