A new strain of MRSA has been discovered in British milk, scientists report today.
The superbug, resistant to antibiotics, has been isolated from samples of milk taken from farms around the country and has also infected humans.
It is the first time MRSA has been found in farm animals in the UK, and has also been identified in Ireland.
Experts said that as virtually all milk sold in Britain is pasteurised, drinking it or eating dairy products was "not a health concern". Meat was also unlikely to be affected, but any MRSA present would be destroyed in cooking anyway.
The "main worry" was that the bacteria might colonise people who work on farms, who might then transmit it to the wider community.
The revelation triggered immediate protests over intensive farming and the use of antibiotics in livestock.
The Soil Association demanded an end to routine antibiotic use in dairy farming and the introduction of comprehensive tests for MRSA of farm animals, farm workers, milk and meat.
The first human case of infection with the new strain was found in Cambridge in 2009. Since then, 12 cases have been identified in Scotland, 14 in England and 24 in Denmark.
Infections with MRSA are treatable, but cause more severe illness, require stronger antibiotics and take longer to resolve.
The discovery is the result of a chance finding by researchers at the University of Cambridge, who were investigating mastitis in cows, an infection of the udder.
Their results, published in The Lancet Infectious Diseases, show cases of the new strain are increasing, but account for less than 1% of all human MRSA detections. Less than 3% of dairy herds are thought to be affected.
Mark Holmes of the Department of Veterinary Medicine at the University of Cambridge, who led the study, said: "It is still not known for certain if cows are infecting people, or people are infecting cows. This is one of the things we will be looking into."
The bug, often found in hospitals, was linked to 1,593 deaths in 2007 but since then the number of suspected fatal cases has fallen dramatically.