A new strain of the MRSA "superbug" has been found in British cows and is believed to be infecting humans.

Environmental campaigners say the new strain has emerged because of the over-use of antibiotics by dairy farmers.

Dr Mark Holmes of Cambridge University, who led the research, said this was a "credible hypothesis".

The researchers, writing in the Lancet Infectious Diseases Journal, say there is no additional health risk from eating milk and dairy products.

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Dr Holmes and his colleague Dr Laura Garcia-Alvarez discovered the new strain while studying a bacterium known to cause mastitis in cows.

They found that, like other MRSA strains, it was resistant to the most commonly used antibiotics. However, the bug was found to be genetically very different.

Subsequent research showed that the strain was also present in humans.

Dr Garcia-Alvarez says that finding a new strain in both in humans and cows is "very worrying".

"Workers on dairy farms are at higher risk of carrying MRSA but we don't yet know if this translates to a higher risk [of them becoming ill]," she said.

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Dr Holmes and Dr Garcia-Alvarez will now investigate the prevalence of the new strain and whether it is more or less harmful than current strains.

They also plan to conduct studies on farms to look for more MRSA strains of this type and explore any potential risks to farm workers.

MRSA is often found in hospitals and was linked to 1,593 deaths in 2007.

Since then the number of suspected fatal cases has fallen dramatically. There were 1,290 in 2008 and 781 in 2009.

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, ARHAI, will be reviewing this issue at their next meeting and will consider potential medical, veterinary and food safety issues."

A Food Standards Agency spokesman said the study did not provide direct evidence that humans were 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," he added.