More cases of a virulent form of E.coli have been found on farms in Wales and South West England, mainly Somerset and Wiltshire, the Soil Association said.
The strain, known as Extended Spectrum Beta Lactamase E.coli, was first discovered in Wales in 2004.
Richard Young, from the Soil Association, said: "Since then, it has been found in 32 farms across the UK."
A spokeswoman for DEFRA confirmed the number of cases and said it was monitoring the situation.
The strain causes urinary infections, of which there were 30,000 human cases last year, which can ultimately be fatal.
Mr Young added: "Studies outside the UK show that it can move [from animals] to humans. But it is hard to track these things.
"The worry is that there is an increasing number of cases in farms."
A spokeswoman for DEFRA said on the farms where the strain had so far been identified there was no human disease.
The Soil Assocation wants to stop animals being sold from any infected farm, and for a review into some antibiotics used in dairy farming which it believes helps the spread of the strain.
DEFRA said it recognised the second concern.