A farmer has spoken out against an undercover animal rights investigator who secretly filmed one of his pigs being beaten to death with an iron bar.
The RSPCA said it had started an inquiry after seeing the "shocking" images of pigs being beaten at Harling Farm, East Harling, Norfolk.
Farmer Stephen Brown said when the Animal Equality activist was filming he was not looking after the pigs.
Animal Equality said the activist's main concern was the pigs' welfare.
The farm has also been criticised by the Red Tractor food mark scheme, which promises the highest standards of animal welfare.
Following the film footage being released, Mr Brown told the BBC: "I was absolutely gutted and mortified. I had no idea anyone was filming anyone.
"It does look bad. I feel some of it has been a bit dramatised. The man concerned taking the film of the pigs he was supposed to be looking after."
He said the pig which was beaten to death with a bar had a broken leg and would normally be shot.
"(Unfortunately the worker) did not have access to the shot gun which I keep at my house and he used a blunt object to kill it," said Mr Brown.
Laura Gough, spokeswoman for Animal Equality UK, said: "I am not surprised the farmer is being defensive by accusing the actual investigator who has exposed his farm.
"He highlighted many occasions of pigs with injuries."
She said she could not understand how Mr Brown could justify the beating to death of an animal.
David Clarke, chief executive of Assured Food Standards - which runs the Red Tractor scheme, told BBC Look East: "What we see in the video is shocking. It has no place in our scheme or in the British farming industry.
"It is really disappointing."
He said the Red Tractor scheme included stringent inspections, but the organisation would have to look at improving these and possibly stop giving notice of inspections to farmers.
A spokesman for the RSPCA said in a statement it was "disgusted by the apparent deliberate animal cruelty being inflicted in the footage, which has been reviewed by one of our farm animals scientists".
"It is one of the worst cases of animal abuse we have seen. Within hours of receiving the footage on 10 February 2012 two RSPCA inspectors and a chief inspector attended the farm to check the welfare of any animals there.
"We are treating this matter extremely seriously and a full and thorough investigation has begun."
Animal rights organisation Animal Equality said the footage was filmed by an undercover reporter between 27 July and 30 September last year.
The 600-acre (242 hectare) arable and mixed farm has 4,000 pigs and has been owned by the Brown family since 1920.