Pet ferrets kept as therapy animals at the UK’s largest children’s prison have been co-opted by managers to kill rats, resulting in a bloody incident and concerns over child and animal welfare.

The unorthodox method of vermin control was waved through last month at HMYOI Wetherby in West Yorkshire following a surge in rat numbers in prison offices and grounds.

According to a union complaint seen by the Guardian, the decision resulted in a boy who looks after ferrets witnessing an “inappropriate and potentially distressing” savaging of a screaming rat.

Another complaint suggests the gored rodent was then “stomped to death” by a senior staff member in front of prison officers.

The incident once again highlights the vermin-ridden conditions in which children and adults are detained within the prison estate across England and Wales.

Mark Fairhurst, the national chair of the Prison Officers Association (POA), called for a reversal of the policy, claiming that the incident happened because private contractors are failing to keep prisons vermin-free.

“This is an unacceptable working practice and should be abolished immediately. This once again proves that the unfit for purpose outsourced maintenance contract needs to be abandoned and brought back in house,” he said.

Animal assisted therapy was introduced more than a decade ago at Wetherby, a former borstal which has served “as a national resource for the most vulnerable children in prison”.

The young offender institution, built around decaying 1950s brick buildings, 1990s blocs and repurposed naval huts, can hold up to 288 boys aged 15 to 18, but at present holds about 100.

Ferrets are kept as part of a therapeutic programme for traumatised children, Prison Service sources said.

But in mid-May, staff and children complained about dozens of rats running around the grounds and inside prison buildings. A team of officers and children who are allowed to take on maintenance duties, called ‘Q Branch’, were given permission to use ferrets to kill the rats.

According to a POA complaint sent to Mark Scott, the institute’s governor, “a rat was cornered and killed within a staff office using domesticated ferrets. This raises significant concerns in relation to animal welfare, health and safety, infection control, and professional conduct.”

“It is particularly concerning that a young individual responsible for the ferrets was present and witnessed the incident. Exposure to such an event is inappropriate and potentially distressing, and warrants review.”

Rather than destroy caught rats by approved pest control methods, a manager advised “stomping on them or throwing them against walls”, the union’s complaint said.

According to a separate complaint sent to the Health and Safety Executive, the ferret “was running freely and chased the rat eventually biting it and holding it still” until a senior staff member “stomped on the rat until it died”. Several staff also witnessed the incident, it was claimed.

Permission to use ferrets to hunt rats was given by a named member of the senior management team, the HSE complaint said.

The incident will not only traumatise child inmates and prison officers but is a “contamination risk” and raises concerns under the Health and Safety at Work Act 1974, the Animal Welfare Act 2006 and the Public Health (Control of Disease) Act 1984, the union said.

While ferrets are often associated with hunting rabbits, some psychologists have said they are playful and responsive animals that can help traumatised teenagers.

The National Ferret Welfare Society said ferreting should always be carried out in “a controlled manner by experienced adults”.

“While we cannot comment on the specific situation without further information, we cannot condone the stamping to death of any animal in any situation,” a spokesperson said.

Rat infestations at prisons across England and Wales are increasingly common. A recent inspection at HMP Manchester, formerly Strangeways, found wings smelling strongly of rat urine, exacerbated by exercise yards covered in litter thrown out of cell windows. At HMP Rochester, prisoners were forced to block gaps under their cell doors with cardboard in an attempt to keep rats out.

In February, a report by HM Inspectorate of Prisons into Wetherby found high levels of violence and self-harm.

Prison Service sources blamed the influx of rats at Wetherby on building work disturbing a rats’ nest. A Youth Custody Service spokesperson said: “This was an isolated incident relating to maintenance works. HMYOI Wetherby has since strengthened its pest control procedures to keep vermin out.”