A gene which makes superbug bacteria resistant to a "last resort" antibiotic has been found in pigs and humans in England and Wales.
The resistance to the colistin antibiotic is considered to be a "major step" towards completely untreatable infections, the Soil Association said.
Public Health England said the risk posed to humans by the mcr-1 gene was "low" but was being monitored closely.
Government scientists re-examined 24,000 samples of bacteria from food and humans in the UK following the discovery of mcr-1 in China in November and found the gene in just 15 samples.
The Soil Association said the mcr-1 was found in E. coli from two pig farms, in stored E. coli from a pig and in three E. coli samples from two human patients, which were also found to be resistant to other antibiotics.
It was also found in 10 human salmonella infections and in salmonella from a single imported sample of poultry meat. The earliest British positive sample was a salmonella from 2012.
Professor Alan Johnson, head of the Department of Healthcare Associated Infection (HCAI) and Antibiotic Resistance at Public Health England, said: "The mcr-1 gene, recently identified as a cause of resistance to the antibiotic colistin, has been found in a very small number of samples of bacteria - 15 out of 24,000, from humans and food tested in the UK.
"Our assessment is that the public health risk posed by this gene is currently considered very low but is subject to ongoing review as more information becomes available.
"The organisms identified can be killed by cooking your food properly and all the bacteria we identified with this gene were responsive to other antibiotics, called carbapenems.
"We will monitor this closely and will provide any further public advice as needed."
The resistance gene has also been found recently in Denmark, France, the Netherlands, Portugal and in several Asian and African countries.
The Soil Association said colistin is frequently used for mass medication of intensively farmed pigs and poultry and scientists believe the resistance gene has spread from farm animals to humans because the antibiotic is used more widely in veterinary medicine than it is in human medicine.
A freedom of information request by the Alliance to Save Our Antibiotics revealed that 837kg of colistin were sold for use in British farm animals last year, while just 300kg was used in human medicine.
Coilin Nunan, scientific adviser to the Alliance to Save Our Antibiotics, said: "Despite scientists saying that resistance to this last-resort antibiotic is likely to be spreading from farm animals to humans, it still remains completely legal in the UK and in most EU countries to routinely feed colistin to large groups of intensively farmed animals, even when no disease has been diagnosed in any of the animals.
"We need the Government, the European Commission and regulatory bodies like the Veterinary Medicines Directorate to respond urgently. The routine preventative use in farming of colistin, and all antibiotics important in human medicine, needs to be banned immediately."