Animal welfare organisations have raised concerns about how much faith shoppers can have in the Red Tractor assurance scheme after animals at a farm it inspected were found to be suffering from infected injuries.

Dozens of pigs at the intensive farm in Lincolnshire – which supplies major retailers including Tesco – were seen with untreated wounds, which critics said suggested “long-term mismanagement on the farm”, where last month workers were shown attacking the animals, in one case with a pitchfork.

Bosses at the Red Tractor scheme, which claims to offer shoppers rigorous standards of farm produce, said then that they had inspected Fir Tree Farm five times in the past year.

But animal welfare campaigners say the footage “brings into question the effectiveness” of the Red Tractor inspections because it also reveals that pigs there were suffering from infected injuries and hernias.

“Many animals were suffering severe pain from untreated wounds, especially deep tail-biting wounds and body scarring,” said a spokeswoman for Animal Equality, which shot the undercover footage.

Four workers were sacked from the farm last month after secret filming exposed more than 100 incidents of them violently and repeatedly kicking pigs in the face and head. The men jabbed the animals repeatedly with metal forks, sprayed marker paint directly up one pig’s nose and slammed gates on animals’ heads.

But Toni Shephard, of Animal Equality, said the painful injuries were overlooked because the violence by workers was so shocking, “but the suffering caused by the infected wounds is equal – if not greater than – the abuse”.

A young pig shows bad scarring from wounds (Animal Equality)

“We filmed multiple pigs with deeply infected tail-biting wounds and severe body scarring in several sheds on the farm in each visit we made,” he said.

“Can consumers really trust the Red Tractor if they didn’t notice this severe suffering?”

Red Tractor scheme chiefs suspended the farm after the violent attacks by workers were exposed, but they say the tail-biting did not warrant suspension.

Animal Equality investigators said they saw the dozens of injured pigs in multiple pens in a large shed marked “growers yard”.

Dr Shephard said boredom in barren conditions or aggression arising from overcrowding was likely to be behind the unhealed wounds: tail injuries would have been caused by pigs biting others’ tails, and the body and neck injuries by repeated mounting of females by males.

Steven McCulloch, a vet and lecturer at the University of Winchester, said the insanitary housing left the pigs at risk of slowly dying from infections from their wounds travelling up their spines.

Many were kept on barren slatted flooring and given no separate area – as they would naturally – to relieve themselves, so the dirt on their skin was their own waste, he said.

In a report on the video footage and photos, Dr McCulloch said: “The pigs lying on slats are dirty. This dirt is likely to be faeces from the slats. These pigs have open skin wounds caused by fighting that will be infected due to the faecal contamination.”

He said the video and photographs demonstrated that the pigs would have been stressed as well as in pain.

Highlighting one animal with multiple wounds seen in photos, he said: “There is no evidence that this pig has been provided with veterinary care. Basic medical treatment for an animal with open wounds would include bathing them in antibacterial solution. This pig is also likely to be bullied by another pig in part due to the poor housing and other factors causing stress.”

Tail stumps had been bitten by other pigs, potentially causing serious infections (Animal Equality)

Dr McCulloch concluded: “It is likely that some pigs will die from severe tail-bite injuries, for instance caused by ascending bacterial infection along the spinal cord causing a systemic infection. There is no evidence from these photographs the pigs are being treated for these injuries.

“Some have deep wounds caused by fighting, almost certainly contaminated by faecal matter from the slatted housing. The pigs have not been moved to a hospital pen.”

A Red Tractor spokesman cited five dates between February last year and February this year on which Fir Tree Farm had been inspected by independent vets, who did not suggest suspending it from the scheme. The tail-biting outbreak was being dealt with by the farmer, and the pigs had been moved into a hospital pen under the care of a specialist vet, he said.

“There has been a very robust, detailed and frequent vet inspection regime at Fir Tree Farm,” the spokesman added.

“The pigs with tail-biting injuries that are on straw in the footage had been removed from the general population of pigs and housed in a hospital pen where they were receiving treatment – which is required in our standards.”

Following the violence by the four workers, a management plan was put in place for the farm, which is owned by Elsham Linc, one of Britain’s largest pig producers.

“I would stress that we would never ignore a tail-biting outbreak. Serious outbreaks may result in suspension,” the Red Tractor spokesman said.

“Tail-biting is a complex issue but the farmer together with the vet must put in place a plan to deal with the problem. If they do not they will be suspended. We closely monitor what action they take to stop it.”

However, Dr Shephard denied the wounded pigs had been in a hospital pen.

“You simply would not have 20 animals or more, some injured and some not, mixed together in a ‘hospital pen’. The vet report also states that the injured animals do not appear to have been isolated,” she said.

She added: “Any animal with a wound that bad should be isolated in a pen alone, because you can clearly see in our video footage that other pigs are chewing on the tails of pigs with injured tails in these crowded pens. So these wounds will never heal.”

She branded Red Tractor’s insistence that the animals were in a hospital pen “a pathetic attempt to try to account for their negligence which caused these animals to endure prolonged suffering”.

But Red Tractor insisted: “In terms of overcrowding, hospital pens are subject to the same spacial allowances as elsewhere throughout the scheme. These are based on the weight of the pig and entirely supported by science. Pigs may also cluster or move around together.”

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Fir Tree Farm said Animal Equality’s claims were not true. A spokesman said in a statement: “The animals shown in this part of the video are in the hospital pen.

“Injured pigs are removed to a hospital pen where we can isolate them and treat any injuries.