Scientists working for a Swiss biotech firm, Alicon, say they have managed to detect prion proteins in the milk of cows, as well as that of humans, sheep and goats, for the first time.
Some prions are known to cause brain diseases, such as Mad Cow disease (BSE) and its human variant, CJD.
It is hoped the breakthrough in Switzerland will help scientists to better understand whether the prion proteins responsible for BSE are also present in the cow's milk.
Prions were recently found in both pasteurised and homogenised milk on supermarket shelves, using new analytical equipment from Alicon, but it remained unclear whether those detected were harmful or not.
"In the case of the prion proteins detected, it is highly likely that they were of a normal variety posing no danger to health," said Dr Ralph Zahn, Alicon's head of research.
But, he warned: "So far there has been no scientific basis for assuming that only 'healthy' prion proteins are present in milk and those causing disease were not."
The Alicon team believes the mere presence of normal prions opens the possibility that disease-carrying prions would also be present in milk from an infected cow.
A rapid test to identify prions in milk is now being developed by the firm, which is a spin-off from the Swiss Federal Institute of Technology.
The possibility of BSE being passed to humans via milk has been a genuine concern for several years.
Some evidence suggests there may be a risk, according to the UK Food Standards Agency (FSA), although no direct data from controlled experiments exists.
Milk is the only product available for public consumption that is derived from BSE-susceptible animals over thirty months of age.