A new strain of MRSA has been found in British milk, indicating that the superbug is spreading through the livestock population and poses a growing threat to human health.

The new strain, MRSA ST398, has been identified in seven samples of bulk milk from five different farms in England.

The discovery, from tests on 1,500 samples, indicates that antibiotic-resistant organisms are gaining an increasing hold in the dairy industry.

The disclosure comes amid growing concern over the use of modern antibiotics on British farms, driven by price pressure imposed by the big supermarket chains. Intensive farming with thousands of animals raised in cramped conditions means infections spread faster and the need for antibiotics is consequently greater.

Three classes of antibiotics rated as “critically important to human medicine” by the World Health Organisation – cephalosporins, fluoroquinolones and macrolides – have increased in use in the animal population by eightfold in the last decade.

The more antibiotics are used, the greater the likelihood that antibiotic-resistant bacteria, such as MRSA, will evolve.

Experts say there is no risk of MRSA infection to consumers of milk or dairy products so long as the milk is pasteurised. The risk comes from farmworkers, vets and abattoir workers, who may become infected through contact with livestock and transmit the bug to others.

The discovery was made by scientists from Cambridge University who first identified MRSA in milk in 2011. They say the latest finding of a different strain is worrying.

Mark Holmes, of the department of veterinary medicine, who led the study, published in Eurosurveillance, said: “This is definitely a worsening situation. In 2011 when we first found MRSA in farm animals, the Department of Environment, Food and Rural Affairs [Defra] initially didn’t believe it. They said we don’t have MRSA in the dairy industry in this country.”

“Now we definitely have MRSA in livestock. What is curious is that it has turned up in dairy cows when in other countries on the Continent it is principally in pigs. Could it be in pigs or poultry in this country? We don’t know.”

The MRSA superbug can cause serious infections in humans which are difficult to treat, require stronger antibiotics, and take longer to resolve. Human cases of infection with the new strain have been found in Scotland and northern England according to Defra, but no details are available.

Dr Holmes said supermarket pressure on farmers to hold down prices was leading to the overuse of antibiotics to prevent cattle getting mastitis, an infection of the udder, that might interrupt the milk supply.

“If farmers were not screwed into the ground by the supermarkets and allowed to get a fair price for their milk they would be able to use fewer antibiotics,” he said.

“Common sense tells us that anything we can do to reduce use of antibiotics will reduce the growth of resistant bugs. We want to wean our farmers off antibiotics and the only way we can do that is with better regulation.”