Scientists in the UK have discovered a new strain of MRSA that appears to spread to humans from cattle and can cause life-threatening illness.

The drug-resistant strain was identified in cows' milk during a study of the udder infection mastitis in dairy herds. It is the first time any form of the MRSA – or methicillin-resistant Staphylococcus aureus – has been found on British farms.

The new strain of the "superbug" has caused a small number of serious blood infections and more minor conditions in people, but health officials have downplayed the risk to the general population because standard tests in UK hospitals should detect the organism.

It is not known how the patients became infected with the new strain, but a likely route would be through contact with infected cattle or people who work with the animals.

Dr Mark Holmes, a veterinary scientist who led the study at the University of Cambridge, said milk from infected cows was safe to drink because the bug, along with other bacteria, was killed by pasteurisation. More than 99% of milk consumed in the UK is pasteurised. The bug was unlikely to survive in unpasteurised cheese, Holmes added.

But the presence of MRSA in the national herd will put farm workers at risk of becoming carriers of the infection and spreading it to the wider community. Early tests suggest the strain has spread to nearly 3% of dairy farms in the UK. A more extensive survey of British farms will begin this summer.

Roughly a third of the human population already carries S. aureus in their noses or on their skin, with around 1% of this being the drug-resistant form MRSA. The bug does not usually cause problems unless it infects broken skin.

The discovery of the new strain has led to calls for a rethink of how antibiotics are used on farms. The drugs are given to treat mastitis and other infections in cattle, but overuse encourages resistant bacterial strains to emerge.

The Cambridge group stumbled on the new strain of MRSA while testing milk from a farm in southwest England in 2007. Further work established that the strain existed at dairy farms across the UK. The same strain was later found among samples of S. aureus taken from people in Scotland, England, Ireland, Denmark and Germany. The oldest came from a Danish patient in 1975, implying the strain has gone unnoticed for at least 35 years.

According to a report in the journal Lancet Infectious Diseases, the bug appears to be on the rise, with the number of human cases in England and Scotland increasing from one in 2002 to nine in 2009 and 12 last year. "Whether we are looking at the beginning of a small peak, or a curve that will continue to rise, we don't know, but it does appear to be rising at the moment," Holmes said.

While the new strain can be detected by standard hospital tests in Britain, it is not picked up by the "gold standard" test used to confirm borderline cases of MRSA, because it has a different genetic make-up to the normal strain. One concern is that hospitals on the continent, which rely only on the latter test, will fail to detect the strain.

A spokesman for Dairy UK said the industry group had met with Holmes and was working with the university to expand its research.

"In the extremely rare cases where infection has occurred, this has been through physical contact, either cattle to cattle or cattle to people, and through cuts or open wounds and not through consumption of milk. The scrupulous hygiene practised by Britain's dairy farmers minimises risks of transmission," a spokesman for Dairy UK said.

Helen Browning, director of the organic food trade group the Soil Association, said the finding highlighted the need to minimise antibiotic use on farms.

"In the relentless drive for increased per animal productivity, and under acute price pressure, dairy systems are becoming ever more antibiotic dependent. We need to get farmers off this treadmill, even if that means that milk has to cost a few pennies more. That would be a very small price to pay for maintaining the efficacy of these life-saving drugs," she said.