Staff working at a chain of commercial vet surgeries have extended their strike, accusing their private-equity-backed owner of underpaying workers and overcharging pet owners as part of the first industrial action to hit the veterinary sector in the UK.

Unionised vets, nurses and support staff at Valley Vets in south Wales, which is owned by one of the largest veterinary corporations in the country, VetPartners, decided last week to stay out until the end of the month, in the latest move in an increasingly bitter dispute.

“The animal care assistants and reception staff at Valley Vets are really poorly paid. They are on either minimum wage or just above it, which in the current climate is untenable,” said Suzanna Hudson-Cooke, chair of the British Veterinary Union (BVU), part of the Unite union.

“VetPartners is a very large company, with an estimated value of about £3bn. They could afford to pay the real living wage of £12 an hour.”

The domestic animal care sector has been progressively taken over by corporations since it was deregulated in 1999.

Almost 60% of UK vet surgeries are now owned by six large companies, which have bought up hundreds of independent practices while often retaining their original names, leaving the public in the dark about the new corporate makeup of the sector.

At least three of the dominant groups, including VetPartners, are owned by private equity investors.

The strike comes as the Competition and Markets Authority continues its investigation into the UK pet market.

The watchdog warned this year that owners may be overpaying for medicines. It is also concerned that large pet care corporations sometimes own many different-looking vet surgeries in local areas, limiting competition and consumer choice.

A union survey of Valley Vets staff found that 80% of them regularly borrowed money to meet basic living costs and 5% had to use food banks.

The BVU also says that vets and other better-paid staff are facing real-terms pay cuts. The union is asking for pay rises for all staff in line with inflation, with the lowest-paid workers given the real living wage.

One animal care assistant, who asked not to be named, said she had been forced to use food banks. “We can’t afford to live. On more than one occasion I’ve had to go to food banks. It’s dehumanising and embarrassing to be put in that position when I’m giving everything I’ve got to the job that I do. VetPartners do not seem to care about that.”

The BVU claimed the treatment fees at Valley Vets had increased by 25% over the last two years. It also said it has often had to point out that medicines are much cheaper online than to buy in the practice.

One vet said high fees had contributed to “horrendous situations” where animals were being put down unnecessarily. “A dog with an acute abdominal illness was hospitalised. If we could have hung on for a few more days, there’s a really good chance it would have made a full recovery. But we ended up putting that dog to sleep because we reached the economic end of the road,” said the vet. “The family felt a mixture of anger, guilt and grief.”

He claimed that VetPartners pressured him to charge customers for every last procedure and treatment, no matter how small. He said he was made to attend seminars about charging and was audited on “charge leakage”, which led to vets being disciplined for providing services to pet owners without a charge.

“We’ve got all this pressure from the bean counters at the top and all you want to do is the best for the patient and the client,” he said.

The animal assistant claimed she was seeing more and more people putting their pets to sleep because they could not afford to pay the fees. “It is devastating. We all got into this career because we care about animals. So to know that animals are suffering because VetPartners are too busy worrying about their bottom line is heartbreaking.”

VetPartners said it was disappointed it had been unable to reach an agreement with the BVU. It said the union’s pay demands would raise its employment costs by 15%, which it claimed would make Valley Vets unsustainable. It said it had prioritised the lowest-paid team members with a 7.27% pay increase, while higher-paid colleagues also received an increase at a lower rate.

A spokesperson said: “Providing the best possible care of pets comes above all else across our veterinary practices. We do not feel that fees paid by our clients can be raised any further to support significant salary increases demanded by the BVU in Unite. Fee increases have not kept pace with increases in costs in recent years, and we have seen profitability fall over the last few years.”