Bacteria that resist the most common antibiotic of last resort - colistin - have been discovered in the UK.
Officials say the threat to human health is low, but is under ongoing review.
Scientists warned the world was on the cusp of a post-antibiotic era when such resistance was discovered in China last month.
Now checks have discovered the same resistance on three farms and in samples of human infections.
When all other antibiotics fail then doctors turn to colistin - that's why it is so important.
Doctors in the UK thought they had three years before colistin-resistance would spread from China to the UK.
But Public Health England and the Animal and Plant Health Agency began testing for it.
Public Health England has gone through the 24,000 bacterial samples it keeps on record from cases between 2012 and 2015.
Colistin-resistance was discovered in fifteen of them, including samples of Salmonella and E. Coli.
The Animal and Plant Health Agency has discovered colistin-resistant bacteria on three pig farms.
The news will not be a massive surprise after similar discoveries in parts of Europe, Asia and Africa.
It raises the prospect of untreatable infections - what is known as the antibiotic apocalypse and threatens to plunge medicine back into the dark ages.
The DNA that gives bacteria resistance to colistin - the mcr-1 gene - can spread rapidly between species.
The concern is that colistin-resistance will now find its way into other superbugs to create infections that doctors cannot treat.
Prof Alan Johnson, from Public Health England, said: "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 Chinese resistance cases were down to overuse of antibiotics in agriculture.
Alliance to Save Our Antibiotics says 837kg of colistin was sold to British farms in 2014.
Coilin Nunan, from the Alliance to Save Our Antibiotics, said: "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."
The Department for Environment, Food and Rural Affairs said colistin made up just 0.2% of the antibiotics used in livestock in the UK.
A spokesperson said: "We are enhancing our surveillance for colistin resistance, and veterinary prescribers have voluntarily updated prescribing guidelines to restrict use of colistin in animals."
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