They identified bacteria able to shrug off the drug of last resort - colistin - in patients and livestock in China.
They said that resistance would spread around the world and raised the spectre of untreatable infections.
Experts said the worrying development needed to act as a global wake-up call.
Bacteria becoming completely resistant to treatment - also known as the antibiotic apocalypse - could plunge medicine back into the dark ages.
Common infections would kill once again, while surgery and cancer therapies, which are reliant on antibiotics, would be under threat.
Key players
Chinese scientists identified a new mutation, dubbed the MCR-1 gene, that prevented colistin from killing bacteria.
The report in the Lancet Infectious Diseases showed resistance in a fifth of animals tested, 15% of raw meat samples and in 16 patients. And the resistance had spread between a range of bacterial strains and species, including E. coli, Klebsiella pneumoniae and Pseudomonas aeruginosa.
There is also evidence that it has spread to Laos and Malaysia.
Prof Timothy Walsh, who collaborated on the study, from the University of Cardiff, told the BBC News website: “All the key players are now in place to make the post-antibiotic world a reality.
“If MRC-1 becomes global, which is a case of when not if, and the gene aligns itself with other antibiotic resistance genes, which is inevitable, then we will have very likely reached the start of the post-antibiotic era.
“At that point if a patient is seriously ill, say with E. coli, then there is virtually nothing you can do.”
And the resistance had spread between a range of bacterial strains and species, including E. coli, Klebsiella pneumoniae and Pseudomonas aeruginosa.
There is also evidence that it has spread to Laos and Malaysia.
Prof Timothy Walsh, who collaborated on the study, from the University of Cardiff, told the BBC News website: “All the key players are now in place to make the post-antibiotic world a reality.
“If MRC-1 becomes global, which is a case of when not if, and the gene aligns itself with other antibiotic resistance genes, which is inevitable, then we will have very likely reached the start of the post-antibiotic era.
“At that point if a patient is seriously ill, say with E. coli, then there is virtually nothing you can do.”
Prof Laura Piddock, from the campaign group Antibiotic Action, said the same antibiotics “should not be used in veterinary and human medicine”.
She told the BBC News website: “Hopefully the post-antibiotic era is not upon us yet. However, this is a wake-up call to the world.”
She argued the dawning of the post-antibiotic era “really depends on the infection, the patient and whether there are alternative treatment options available” as combinations of antibiotics may still be effective.
A commentary in the Lancet concluded the “implications [of this study] are enormous” and unless something significant changes, doctors would “face increasing numbers of patients for whom we will need to say, Sorry, there is nothing I can do to cure your infection.”
More Information on BBC