All three Holsteins cows in Japan diagnosed with BSE were fed with a milk substitute from the same factory, the Gunma prefecture government revealed on Saturday.

The ministry had hoped that because they were all born in the spring of 1996, the source of their contamination could be found by checking their feed.

It is as yet unclear if this finding will identify the source of the mad cow infection, although experts have said that cases exist in Britain of cows being contaminated with BSE after consuming infected milk substitute containing meat-and-bone meal (MBM).

The Japanese milk substitute manufacturer is adamant however that the product did not contain MBM.