A new superbug that is resistant to even the most powerful antibiotics has entered UK hospitals, experts warn.
They say bacteria that make an enzyme called NDM-1 have travelled back with NHS patients who went abroad to countries like India and Pakistan for treatments such as cosmetic surgery.
Although there have only been about 50 cases identified in the UK so far, scientists fear it will go global.
Tight surveillance and new drugs are needed says Lancet Infectious Diseases.
NDM-1 can exist inside different bacteria, like E.coli, and it makes them resistant to one of the most powerful groups of antibiotics - carbapenems.
These are generally reserved for use in emergencies and to combat hard-to-treat infections caused by other multi-resistant bacteria.
And experts fear NDM-1 could now jump to other strains of bacteria that are already resistant to many other antibiotics.
Ultimately, this could produce dangerous infections that would spread rapidly from person to person and be almost impossible to treat.
At least one of the NDM-1 infections the researchers analysed was resistant to all known antibiotics.
Similar infections have been seen in the US, Canada, Australia and the Netherlands and international researchers say that NDM-1 could become a major global health problem.
Infections have already been passed from patient to patient in UK hospitals.
The way to stop NDM-1, say researchers, is to rapidly identify and isolate any hospital patients who are infected.
Normal infection control measures, such as disinfecting hospital equipment and doctors and nurses washing their hands with antibacterial soap, can stop the spread.
And currently, most of the bacteria carrying NDM-1 have been treatable using a combination of different antibiotics.
But the potential of NDM-1 to become endemic worldwide is "clear and frightening", say the researchers in The Lancet infectious diseases paper.
The research was carried out by experts at Cardiff University, the Health Protection Agency and international colleagues.
Dr David Livermore, one of the researchers and who works for the UK's Health Protection Agency (HPA), said: "There have been a number of small clusters within the UK, but far and away the greater number of cases appear to be associated with travel and hospital treatment in the Indian subcontinent.
"This type of resistance has become quite widespread there.
"The fear would be that it gets into a strain of bacteria that is very good at being transmitted between patients."
He said the threat was a serious global public health problem as there are few suitable new antibiotics in development and none that are effective against NDM-1.
The Department of Health has already put out an alert on the issue, he said.
"We issue these alerts very sparingly when we see new and disturbing resistance."
The National Resistance Alert came in 2009 after the HPA noted an increasing number of cases - some fatal - emerging in the UK.
The Lancet study looked back at some of the NDM-1 cases referred to the HPA up to 2009 from hospitals scattered across the UK.
At least 17 of the 37 patients they studied had a history of travelling to India or Pakistan within the past year, and 14 of them had been admitted to a hospital in these countries - many for cosmetic surgery.
For some of the patients the infection was mild, while others were seriously ill, and some with blood poisoning.
A Department of Health spokeswoman said: "We are working with the HPA on this issue.
"Hospitals need to ensure they continue to provide good infection control to prevent any spread, consider whether patients have recently been treated abroad and send samples to HPA for testing.
"So far there has only been a small number of cases in UK hospital patients. The HPA is continuing to monitor the situation and we are investigating ways of encouraging the development of new antibiotics with our European colleagues."
The Welsh Assembly Government said it would be "fully considering" the report.
"The NHS in Wales is used to dealing with multi-resistant bacteria using standard microbiological approaches, and would deal with any new bacteria in a similar way," said a spokesperson.