A new strain of the MRSA "superbug" has been found in the milk of British cows as well as swab samples taken from humans.
Experts have ruled out any general threat to the safety of milk or dairy products, but they point to "circumstantial" evidence of the bacteria passing between cattle and the human population.
However, the findings, published in the journal The Lancet Infectious Diseases, have fuelled controversy over intensive farming methods and the way antibiotics are used to protect livestock.
The Soil Association called for a complete ban on routine use of the drugs, which is said to promote the development of drug-resistant bacteria. It also urged an end to the continuing economic pressure on farmers to cut costs and maintain low prices.
Helen Browning, director of the Soil Association, said: "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."
The Cambridge University veterinary scientist who led the research spoke of the "tremendous financial pressure" placed on farmers by the purchasing power of big supermarkets. Dr Mark Holmes and his team stumbled on the new MRSA bug while investigating mastitis, a disease which affects dairy cows.
MRSA, or methicillin-resistant Staphylococcus aureus, is a drug-resistant form of a usually harmless common bacterium which can be deadly when it infects wounds.
Alarm bells rang over the new strain when it showed itself to be drug-resistant yet impossible to identify using standard molecular tests. The strain turned out to have a key gene for antibiotic resistance, called mecA, which was different from the one for "normal" MRSA.
A Department of Health spokesman said: "From the available evidence, we understand this new form of MRSA is rare in the UK and is not causing infections in humans. However our expert committee (Advisory Committee on Antimicrobial Resistance and Healthcare Associated Infections) will be reviewing this issue at their next meeting and will consider potential medical, veterinary and food safety issues."
A spokesman from the Food Standards Agency said: "The data from this study does not provide direct evidence that humans are being infected with MRSA from cattle. The risk of contracting this new strain of MRSA through drinking milk is extremely low because the vast majority of cows' milk is pasteurised and the pasteurisation process destroys all types of MRSA. Additionally most of the samples of the new strain of MRSA found were in milk from cows with udder infections. Milk from these cows is banned for human consumption."